Dynamo support for autograd.Function w/ once_differentiable (#108686)
Fixes #106893
There are two main changes:
- Before this PR, the function returned by once_differentiable was
included in skipfiles (because its .co_code is
torch/autograd/function.py). This PR adds a mechanism to tell Dynamo
to inline a function, no matter if it is included in skipfiles.
- A bugfix: when we are introspecting the backward, we need to turn the
grad mode off. This is to accurately model the eager-mode semantics:
In eager-mode PyTorch, if second-order gradients were not requested, then
the grad mode is off. torch.compile does not work with higher-order
gradients and just assumes we do first-order gradients, so this is OK.
Test Plan:
- new test
Differential Revision: [D49064185](https://our.internmc.facebook.com/intern/diff/D49064185)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108686
Approved by: https://github.com/voznesenskym
