dynamo: wrap graph break inst in try except block - with context manager setup/teardown (#94758)
Replacement to https://github.com/pytorch/pytorch/pull/94672.
Follow up to https://github.com/pytorch/pytorch/pull/94137.
We simply replace the set grad mode try except blocks with one for a more generic contextmanager (using `__enter__` and `__exit__`), storing the context manager into a `symbolic_local` for the duration of the try block.
(see https://github.com/pytorch/torchdynamo/issues/207 for the original motivation)
This allows us to handle calling inner functions with graph breaks for any arbitrarily deep nesting of live context managers subclassing `AbstractContextManager`. (see tests)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94758
Approved by: https://github.com/yanboliang
