Decouple PT2 dynamic shapes from the functorch setting (#94469)
The functorch setting still exists, but now it is no longer necessary:
we infer use of Python dispatcher by checking if the ambient
FakeTensorMode has a ShapeEnv or not. The setting still exists,
but it is for controlling direct AOTAutograd use now; for PT2,
it's sufficient to use torch._dynamo.config.dynamic_shapes.
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94469
Approved by: https://github.com/Chillee, https://github.com/voznesenskym, https://github.com/jansel
