How do I compute/check/understand gradients of inplace ReLU? I did clone() to bypass “a leaf Variable that requires grad has been used in an in-place operation”. Gradient should obviously be 0, but I get 1.
import torch
import torch.nn.functional as F
# not in place
x = torch.tensor([-1.0]).requires_grad_().clone()
print(torch.autograd.grad(F.relu(x), (x, ))[0])
# tensor([0.])
# inplace
x = torch.tensor([-1.0]).requires_grad_().clone()
print(torch.autograd.grad(F.relu_(x), (x, ))[0])
# tensor([1.])
My final usecase is testing some ResNet-like code like following:
def residual_activation_inplace(x, residual, activation, inplace):
for r in residual:
x += r
return activation(x, inplace = inplace)
x = torch.rand(3, 4)
residual = [torch.rand(3, 4), torch.rand(3, 4)]
y = residual_activation_inplace(x, residual, F.relu, inplace = True)
I am confused by the gradient of inplace version ReLU until I found this thread. However, it is still unclear how the backward performs in inplace ReLU.
we can see the inplace ReLU (relu_) replies on the output tensor (result) for its back-propogation, however, this tensor has already been modified to non-negative range. It is confusing how to passing back gradient as we lost the sign of the tensor.
See example below, for inplace ReLU, the saved_tensors in the backward function is non-negative
class MyReLU(torch.autograd.Function):
“”"
We can implement our own custom autograd Functions by subclassing
torch.autograd.Function and implementing the forward and backward passes
which operate on Tensors.
“”" @staticmethod
def forward(ctx, input):
“”"
In the forward pass we receive a Tensor containing the input and return
a Tensor containing the output. ctx is a context object that can be used
to stash information for backward computation. You can cache arbitrary
objects for use in the backward pass using the ctx.save_for_backward method.
“”"
ctx.save_for_backward(input)
return input.clamp(min=0) @staticmethod
def backward(ctx, grad_output):
“”"
In the backward pass we receive a Tensor containing the gradient of the loss
with respect to the output, and we need to compute the gradient of the loss
with respect to the input.
“”"
input, = ctx.saved_tensors
grad_input = grad_output.clone()
grad_input[input < 0] = 0 # for inplace version, grad_input = grad_output, as input is modified into non-negative range?
return grad_input
Thus, the only way for correct inplace ReLU backward propogration is to save another tensor which indicate the sign of the input (flag = input < 0)?
In other word, for inplace ReLU, a flag (e. g. flag = input < 0) tensor is saved for backward, rather than the input/output tensor?
The gradient we want to compute here is indeed: 1 if input > 0 and 0 if inputs <= 0.
The nice thing is that inputs <= 0 <=> relu(inputs) = 0.
So we can actually compute the gradient based on the result with grad_input[result == 0] = 0 (or with <=, that would give the same result as result >=0).