[JIT] Add torch._C._jit submodule (#52910)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52910
**Summary**
PR #52158 tried to move all JIT bindings from `torch._C` to a new
submodule `torch._C._jit`, but that...did not go well. This pull request
adds the new `torch._C._jit` submodule, but does not migrate the
existing bindings. Instead, it adds a unit test that fails if any new
bindings are added to `torch._C`. A comment in the test instructs
developers to add their new binding to the allowlist if it really should
be in `torch._C`, or to add it to the appropriate submodule (e.g
`torch._C._jit`, for example). The idea is to prevent the issue
described in #51691 from getting *worse* if it cannot be fixed.
**Test Plan**
Continuous integration.
**Fixes**
This commit fixes #51691.
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D26698373
Pulled By: SplitInfinity
fbshipit-source-id: ec9f5426051227a513d4fd09512b624420e0100b
