Optimize nn.Module __call__ fast path for dynamo (#95931)
This PR optimizes the guards overhead introduced by dynamo tracing module forward hooks.
It can and maybe should be followed by a wider change proposed by @voznesenskym to optimize specialized nnmodules by 'observing' any user mutations and directly invalidating the root guard, obviating the need to install other nnmodule guards. (But this observer change seems more involved...)
Idea: maintain a flag, and keep it up to date whenever adding or
removing hooks. Use the flag rather than dict checks to enter the call fast path.
- need to extend RemovableHandle to keep a ref to nnModule so it can update the flag on removal.
- also need to handle the flag in ScriptModule which still uses the python call impl when called from python.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95931
Approved by: https://github.com/ezyang, https://github.com/voznesenskym
