Hook Programming

Debugging deep learning networks is a big task for every practitioner in the field of deep learning. Since the deep learning network hides the input and output data as well as the inverse gradient of the intermediate layer operators, only the gradient of the network input data (feature quantity and weight) is provided, resulting in the inability to accurately sense the data changes of the intermediate layer operators, which reduces the debugging efficiency. In order to facilitate users to debug the deep learning network accurately and quickly, MindSpore designes Hook function in dynamic graph mode. Using Hook function can capture the input and output data of intermediate layer operators as well as the reverse gradient.

Currently, five forms of Hook functions are provided in dynamic graph mode: HookBackward operator and register_forward_pre_hook, register_forward_hook, register_backward_pre_hook, register_backward_hook functions registered on Cell objects.

HookBackward Operator

HookBackward implements the Hook function in the form of an operator. The user initializes a HookBackward operator and places it at the location in the deep learning network where the gradient needs to be captured. In the forward execution of the network, the HookBackward operator outputs the input data as is without any modification. When the network back propagates the gradient, the Hook function registered on HookBackward will capture the gradient back propagated to this point. The user can customize the operation on the gradient in the Hook function, such as printing the gradient, or returning a new gradient.

The sample code is as follows:

import mindspore as ms
from mindspore import ops

ms.set_context(mode=ms.PYNATIVE_MODE)

def hook_fn(grad_out):
    """Print Gradient"""
    print("hook_fn print grad_out:", grad_out)

hook = ops.HookBackward(hook_fn)
def hook_test(x, y):
    z = x * y
    z = hook(z)
    z = z * y
    return z

def net(x, y):
    return ms.grad(hook_test, grad_position=(0, 1))(x, y)

output = net(ms.Tensor(1, ms.float32), ms.Tensor(2, ms.float32))
print("output:", output)

hook_fn print grad_out: (Tensor(shape=[], dtype=Float32, value= 2),)
output: (Tensor(shape=[], dtype=Float32, value= 4), Tensor(shape=[], dtype=Float32, value= 4))

For more descriptions of the HookBackward operator, refer to the API documentation.

register_forward_pre_hook Function in Cell Object

The user can use the register_forward_pre_hook function on the Cell object to register a custom Hook function to capture data that is passed to that Cell object. This function does not work in static graph mode and inside functions modified with @jit. The register_forward_pre_hook function takes the Hook function as an input and returns a handle object that corresponds to the Hook function. The user can remove the corresponding Hook function by calling the remove() function of the handle object. Each call to the register_forward_pre_hook function returns a different handle object. Hook functions should be defined in the following way.

def forward_pre_hook_fn(cell, inputs):
    print("forward inputs: ", inputs)

Here cell is the Cell object, and inputs are the data passed forward to the Cell object. Therefore, the user can use the register_forward_pre_hook function to capture the positive input data of a particular Cell object in the network. The user can customize the operations on the input data in the Hook function, such as viewing, printing data, or returning new input data to the current Cell object. If the original input data of the Cell object is computed in the Hook function and then returned as new input data, these additional computation operations will act on the backpropagation of the gradient at the same time.

The sample code is as follows:

import numpy as np
import mindspore as ms
import mindspore.nn as nn

ms.set_context(mode=ms.PYNATIVE_MODE)

def forward_pre_hook_fn(cell, inputs):
    print("forward inputs: ", inputs)
    input_x = inputs[0]
    return input_x

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.relu = nn.ReLU()
        self.handle = self.relu.register_forward_pre_hook(forward_pre_hook_fn)

    def construct(self, x, y):
        x = x + y
        x = self.relu(x)
        return x

net = Net()
grad_net = ms.grad(net, grad_position=(0, 1))

x = ms.Tensor(np.ones([1]).astype(np.float32))
y = ms.Tensor(np.ones([1]).astype(np.float32))

output = net(x, y)
print(output)
gradient = grad_net(x, y)
print(gradient)
net.handle.remove()
gradient = grad_net(x, y)
print(gradient)

forward inputs:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
[2.]
forward inputs:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
(Tensor(shape=[1], dtype=Float32, value= [ 1.00000000e+00]), Tensor(shape=[1], dtype=Float32, value= [ 1.00000000e+00]))
(Tensor(shape=[1], dtype=Float32, value= [ 1.00000000e+00]), Tensor(shape=[1], dtype=Float32, value= [ 1.00000000e+00]))

If the user returns the newly created data directly in the Hook function, instead of returning the data obtained from the original input data after calculation, then the back propagation of the gradient will be cut off on that Cell object.

The sample code is as follows:

import numpy as np
import mindspore as ms
import mindspore.nn as nn

ms.set_context(mode=ms.PYNATIVE_MODE)

def forward_pre_hook_fn(cell, inputs):
    print("forward inputs: ", inputs)
    return ms.Tensor(np.ones([1]).astype(np.float32))

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.relu = nn.ReLU()
        self.handle = self.relu.register_forward_pre_hook(forward_pre_hook_fn)

    def construct(self, x, y):
        x = x + y
        x = self.relu(x)
        return x

net = Net()
grad_net = ms.grad(net, grad_position=(0, 1))

x = ms.Tensor(np.ones([1]).astype(np.float32))
y = ms.Tensor(np.ones([1]).astype(np.float32))

gradient = grad_net(x, y)
print(gradient)

forward inputs:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
(Tensor(shape=[1], dtype=Float32, value= [ 0.00000000e+00]), Tensor(shape=[1], dtype=Float32, value= [ 0.00000000e+00]))

To avoid running failure when scripts switch to graph mode, it is not recommended to call the register_forward_pre_hook function and the remove() function of the handle object in the construct function of the Cell object. In dynamic graph mode, if the register_forward_pre_hook function is called in the construct function of the Cell object, the Cell object will register a new Hook function every time it runs.

For more information about the register_forward_pre_hook function of the Cell object, refer to the API documentation.

register_forward_hook Function of Cell Object

The user can use the register_forward_hook function on the Cell object to register a custom Hook function that captures the data passed forward to the Cell object and the output data of the Cell object. This function does not work in static graph mode and inside functions modified with @jit. The register_forward_hook function takes the Hook function as an input and returns a handle object that corresponds to the Hook function. The user can remove the corresponding Hook function by calling the remove() function of the handle object. Each call to the register_forward_hook function returns a different handle object. Hook functions should be defined in the following way.

The sample code is as follows:

def forward_hook_fn(cell, inputs, outputs):
    print("forward inputs: ", inputs)
    print("forward outputs: ", outputs)

Here cell is the Cell object, inputs is the forward input data to the Cell object, and outputs is the forward output data of the Cell object. Therefore, the user can use the register_forward_hook function to capture the forward input data and output data of a particular Cell object in the network. Users can customize the operations on input and output data in the Hook function, such as viewing, printing data, or returning new output data. If the original output data of the Cell object is computed in the Hook function and then returned as new output data, these additional computation operations will act on the back propagation of the gradient at the same time.

The sample code is as follows:

import numpy as np
import mindspore as ms
import mindspore.nn as nn

ms.set_context(mode=ms.PYNATIVE_MODE)

def forward_hook_fn(cell, inputs, outputs):
    print("forward inputs: ", inputs)
    print("forward outputs: ", outputs)
    outputs = outputs + outputs
    return outputs

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.relu = nn.ReLU()
        self.handle = self.relu.register_forward_hook(forward_hook_fn)

    def construct(self, x, y):
        x = x + y
        x = self.relu(x)
        return x

net = Net()
grad_net = ms.grad(net, grad_position=(0, 1))

x = ms.Tensor(np.ones([1]).astype(np.float32))
y = ms.Tensor(np.ones([1]).astype(np.float32))

gradient = grad_net(x, y)
print(gradient)
net.handle.remove()
gradient = grad_net(x, y)
print(gradient)

forward inputs:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
forward outputs:  [2.]
(Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]), Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]))
(Tensor(shape=[1], dtype=Float32, value= [ 1.00000000e+00]), Tensor(shape=[1], dtype=Float32, value= [ 1.00000000e+00]))

If the user returns the newly created data directly in the Hook function, instead of returning new output data that is obtained after the original output data is calculated, then the back propagation of the gradient will cut off on that Cell object, which can be seen in the use case illustration of the register_forward_pre_hook function. To avoid running failure when the script switches to graph mode, it is not recommended to call the register_forward_hook function in the construct function of the Cell object and the remove() function of the handle object. In dynamic graph mode, if the register_forward_hook function is called in the construct function of the Cell object, the Cell object will register a new Hook function every time it runs.

For more information about the register_forward_hook function of the Cell object, please refer to the API documentation.

register_backward_pre_hook Function of Cell Object

The user can use the register_backward_pre_hook function on the Cell object to register a custom Hook function that captures the gradient associated with the Cell object when the network is back propagated. This function does not work in graph mode or inside functions modified with @jit. The register_backward_pre_hook function takes the Hook function as an input and returns a handle object that corresponds to the Hook function. The user can remove the corresponding Hook function by calling the remove() function of the handle object. Each call to the register_backward_pre_hook function will return a different handle object.

Unlike the custom Hook function used by the HookBackward operator, the inputs of the Hook function used by register_backward_pre_hook contains cell, which represents the information of the Cell object, the gradient passed to the Cell object in reverse of the Cell object.

The sample code is as follows:

def backward_hook_pre_function(cell, grad_output):
    print(grad_output)

Here cell is the information of the Cell object, grad_output is the gradient passed to the Cell object when the network is back-propagated. Therefore, the user can use the register_backward_pre_hook function to capture the backward input gradients of a particular Cell object in the network. The user can customize the operations on the gradient in the Hook function, such as viewing, printing the gradient, or returning the new input gradient. If you need to return the new input gradient in the Hook function, the return value must be in the form of tuple.

The sample code is as follows:

import numpy as np
import mindspore as ms
import mindspore.nn as nn

ms.set_context(mode=ms.PYNATIVE_MODE)

def backward_hook_pre_function(cell, grad_output):
    print(grad_output)

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.conv = nn.Conv2d(1, 2, kernel_size=2, stride=1, padding=0, weight_init="ones", pad_mode="valid")
        self.bn = nn.BatchNorm2d(2, momentum=0.99, eps=0.00001, gamma_init="ones")
        self.handle = self.bn.register_backward_pre_hook(backward_hook_pre_function)
        self.relu = nn.ReLU()

    def construct(self, x):
        x = self.conv(x)
        x = self.bn(x)
        x = self.relu(x)
        return x

net = Net()
grad_net = ms.grad(net)
output = grad_net(ms.Tensor(np.ones([1, 1, 2, 2]).astype(np.float32)))
print(output)
net.handle.remove()
output = grad_net(ms.Tensor(np.ones([1, 1, 2, 2]).astype(np.float32)))
print("-------------\n", output)

(Tensor(shape=[1, 2, 1, 1], dtype=Float32, value=
[[[[ 1.00000000e+00]],
  [[ 1.00000000e+00]]]]),)
[[[[1.99999 1.99999]
   [1.99999 1.99999]]]]
-------------
 [[[[1.99999 1.99999]
   [1.99999 1.99999]]]]

To avoid running failure when the scripts switch to graph mode, it is not recommended to call the register_backward_pre_hook function and the remove() function of the handle object in the construct function of the Cell object. In PyNative mode, if the register_backward_pre_hook function is called in the construct function of the Cell object, the Cell object will register a new Hook function every time it runs.

For more information about the register_backward_pre_hook function of the Cell object, please refer to the API documentation.

register_backward_hook Function of Cell Object

The user can use the register_backward_hook function on the Cell object to register a custom Hook function that captures the gradient associated with the Cell object when the network is back propagated. This function does not work in graph mode or inside functions modified with @jit. The register_backward_hook function takes the Hook function as an input and returns a handle object that corresponds to the Hook function. The user can remove the corresponding Hook function by calling the remove() function of the handle object. Each call to the register_backward_hook function will return a different handle object.

Unlike the custom Hook function used by the HookBackward operator, the inputs of the Hook function used by register_backward_hook contains cell, which represents the information of the Cell object, the gradient passed to the Cell object in reverse, and the gradient of the reverse output of the Cell object.

The sample code is as follows:

def backward_hook_function(cell, grad_input, grad_output):
    print(grad_input)
    print(grad_output)

Here cell is the information of the Cell object, grad_input is the gradient of the reverse output of the Cell object. grad_output is the gradient passed to the Cell object when the network is back-propagated, which corresponds to the reverse output gradient of the next operator in the forward process. Therefore, the user can use the register_backward_hook function to capture the backward input and backward output gradients of a particular Cell object in the network. The user can customize the operations on the gradient in the Hook function, such as viewing, printing the gradient, or returning the new output gradient. If you need to return the new output gradient in the Hook function, the return value must be in the form of tuple.

The sample code is as follows:

import numpy as np
import mindspore as ms
import mindspore.nn as nn

ms.set_context(mode=ms.PYNATIVE_MODE)

def backward_hook_function(cell, grad_input, grad_output):
    print(grad_input)
    print(grad_output)

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.conv = nn.Conv2d(1, 2, kernel_size=2, stride=1, padding=0, weight_init="ones", pad_mode="valid")
        self.bn = nn.BatchNorm2d(2, momentum=0.99, eps=0.00001, gamma_init="ones")
        self.handle = self.bn.register_backward_hook(backward_hook_function)
        self.relu = nn.ReLU()

    def construct(self, x):
        x = self.conv(x)
        x = self.bn(x)
        x = self.relu(x)
        return x

net = Net()
grad_net = ms.grad(net)
output = grad_net(ms.Tensor(np.ones([1, 1, 2, 2]).astype(np.float32)))
print(output)
net.handle.remove()
output = grad_net(ms.Tensor(np.ones([1, 1, 2, 2]).astype(np.float32)))
print("-------------\n", output)

(Tensor(shape=[1, 2, 1, 1], dtype=Float32, value=
[[[[ 9.99994993e-01]],
  [[ 9.99994993e-01]]]]),)
(Tensor(shape=[1, 2, 1, 1], dtype=Float32, value=
[[[[ 1.00000000e+00]],
  [[ 1.00000000e+00]]]]),)
[[[[1.99999 1.99999]
   [1.99999 1.99999]]]]
-------------
 [[[[1.99999 1.99999]
   [1.99999 1.99999]]]]

To avoid running failure when the scripts switch to graph mode, it is not recommended to call the register_backward_hook function and the remove() function of the handle object in the construct function of the Cell object. In PyNative mode, if the register_backward_hook function is called in the construct function of the Cell object, the Cell object will register a new Hook function every time it runs.

For more information about the register_backward_hook function of the Cell object, please refer to the API documentation.

Using the multiple hook function of Cell Object

When the register_backward_pre_hook function, the register_backward_hook function, the register_forward_pre_hook function, and the register_forward_hook function act on the same Cell object at the same time, if the register_forward_pre_hook and the register_forward_hook functions add other operators for data processing, these new operators will participate in the forward calculation of the data before or after the execution of the Cell object, but the backward gradient of these new operators is not captured by the register_backward_pre_hook function or the register_backward_hook function. The Hook function registered in register_backward_pre_hook only captures the input gradients of the original Cell object. The Hook function registered in register_backward_hook only captures the input and output gradients of the original Cell object.

The sample code is as follows:

import numpy as np
import mindspore as ms
import mindspore.nn as nn

ms.set_context(mode=ms.PYNATIVE_MODE)

def forward_pre_hook_fn(cell, inputs):
    print("forward inputs: ", inputs)
    input_x = inputs[0]
    return input_x

def forward_hook_fn(cell, inputs, outputs):
    print("forward inputs: ", inputs)
    print("forward outputs: ", outputs)
    outputs = outputs + outputs
    return outputs

def backward_pre_hook_fn(cell, grad_output):
    print("grad input: ", grad_output)

def backward_hook_fn(cell, grad_input, grad_output):
    print("grad input: ", grad_output)
    print("grad output: ", grad_input)

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.relu = nn.ReLU()
        self.handle = self.relu.register_forward_pre_hook(forward_pre_hook_fn)
        self.handle2 = self.relu.register_forward_hook(forward_hook_fn)
        self.handle3 = self.relu.register_backward_pre_hook(backward_pre_hook_fn)
        self.handle4 = self.relu.register_backward_hook(backward_hook_fn)

    def construct(self, x, y):
        x = x + y
        x = self.relu(x)
        return x

net = Net()
grad_net = ms.grad(net, grad_position=(0, 1))
gradient = grad_net(ms.Tensor(np.ones([1]).astype(np.float32)), ms.Tensor(np.ones([1]).astype(np.float32)))
print(gradient)

forward inputs:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
forward inputs:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
forward outputs:  [2.]
grad input:  (Tensor(shape=[1], dtype=Float32, value= [ 1.00000000e+00]),)
grad input:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
grad output:  (Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]),)
(Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]), Tensor(shape=[1], dtype=Float32, value= [ 2.00000000e+00]))

Here grad_output is the gradient passed to self.relu when the gradient is back-propagated, not the gradient of the new Add operator in the forward_hook_fn function. Here grad_input is the reverse output gradient of the self.relu when the gradient is back-propagated, not the reverse output gradient of the new Add operator in the forward_pre_hook_fn function. The register_forward_pre_hook and register_forward_hook functions work before and after the execution of the Cell object and do not affect the gradient capture range of the reverse Hook function on the Cell object.