Add mechanism to disable the "saved tensors hooks" feature (#85553)
The rationale for this is that functorch doesn't work with saved
variable hooks at the moment or checkpointing and we need some way to
disable it.
Concretely:
- there's a context manager that does the disabling
- this feature is disabled on a thread-local basis
- one can set an error message or use the default error message that
says the feature has been disabled
Since it is thread local I needed to update ATen/ThreadLocalState. To
make things nicer, this PR refactors all the "saved tensors hooks"
related TLS things into a single struct.
Test Plan:
- new test
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85553
Approved by: https://github.com/soulitzer
