We know that we can implement our own custom autograd functions by subclassing torch.autograd.Function and implementing the forward and backward passes which operate on Tensors. As shown in https://pytorch.org/docs/stable/autograd.html#function

class Exp(Function):
@staticmethod
def forward(ctx, i):
result = i.exp()
ctx.save_for_backward(result)
return result
@staticmethod
def backward(ctx, grad_output):
result, = ctx.saved_tensors
return grad_output * result

The question is how can I call the backward function of operations in torch.nn.functional? Is this possible? And what is the simplest way?
For example, I want to call the backward function of torch.nn.functional.softmax and get the Jacobian matrix. I have tried torch.autograd.functional.jacobian, but it is experimental now and gets a wrong result when I apply it to softmax and a 3D tensor. Is there an existing backward function that I can directly call?

The functional methods in torch.nn are not all elementary functions for the autograd. So there isn’t a single backward function to call for them.
What you can do though is get the gradient with the regular autograd:

# inp that requires_grad=True and grad_output that match what you want to compute.
# If you want a full jacobian, you will need a for loop to reconstruct it line by line.
out = functional.softmax(inp)
grad = torch.autograd.grad(out, inp, grad_output)

What is the issue you have with functional.jacobian? What do you mean by wrong result?

Hi albanD,
Thank you for your answer and torch.autograd.grad indeed works for me!
I checked the docs of functional.jacobian again, and I found that I misunderstood the use of it. After I change the code, the result of functional.jacobian is all good now.