This PR adds support for `enable_grad`/`no_grad`/`autocast` context managers getting properly traced in `pre_dispatch` tracing. The stuff in this PR includes:
- I added a torch function mode that runs during make_fx pre_dispatch tracing, `ProxyTorchFunctionMode`. It directly intercepts the torch ops that run during the above context managers, and adds them to the current graph instead of executing them
- `enable_grad` and `no_grad` currently desugar into `torch._C.set_grad_enabled(bool)`, but this API isn't currently overrideable by torch function so I added the ability to interpose there
- the `torch.amp` context managers don't currently have a nice equivalent, like `set_autocast_enabled(state)`, so I ended up adding two new API's: `torch.amp._set_autocast_enabled` and `torch.amp._set_autocast_disabled`. If you look at how the context manager is implemented, it ends up calling several different state-changing functions, some of which depend on the backend - so I figured that it would be cleaner just to add a new API (that should probably only be used by tracing) - but open to feedback
- I added a new dynamo backend, `compile(backend="pre_dispatch_eager")`. When pre_dispatch tracing becomes always-on in inductor, it will be another potential surface for bugs. I also added a test file for it (`test/dynamo/test_pre_dispatch.py`).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103024
Approved by: https://github.com/ezyang
