Fix typing for autograd.Function with ctx-less forward (#122167)
Previously, typing an autograd.Function like the following would lead to
a mypy error (which expects the first arg to forward to be named `ctx`).
This PR fixes that by deleting the ctx arg.
```py
class MySin(torch.autograd.Function):
@staticmethod
def forward(x: torch.Tensor) -> torch.Tensor:
return x.sin()
@staticmethod
def setup_context(*args, **kwargs):
pass
@staticmethod
def backward(ctx, grad):
if grad.stride(0) > 1:
return grad.sin()
return grad.cos()
```
Test Plan:
- tested locally (I don't know how to put up a test in CI for this).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/122167
Approved by: https://github.com/soulitzer
