Add a stable TORCH_LIBRARY to C shim (#148124)

This PR adds two main parts:
- shim.h stable C APIs into torch::Library APIs
- a higher level API in torch/csrc/stable/library.h that calls into this shim.h + otherwise is self contained

Goal: custom kernel writers should be able to call the apis in the directories above in order to register their library in a way that allows their custom extension to run with a different libtorch version than it was built with.

Subplots resolved:

- Do we want a whole separate StableLibrary or do we want to freeze torch::Library and add `m.stable_impl(cstring, void (*fn)(void **, int64_t, int64_t)` into it
    - Yes, we want a separate StableLibrary. We cannot freeze Library and it is NOT header only.
- Should I use unint64_t as the common denominator instead of void* to support 32bit architectures better?
    -  Yes, and done
- Should I add a stable `def` and `fragment` when those can be done in python?
    - I think we do want these --- and now they're done
- Where should library_stable_impl.cpp live? -- no longer relevant
- I need some solid test cases to make sure everything's going ok. I've intentionally thrown in a bunch of random dtypes into the signature, but I still haven't tested returning multiple things, returning nothing, complex dtypes, etc.
    - Have since tested all the torch library endpoints. the others can be tested in a followup to separate components that need to be in shim.h vs can be added later

Pull Request resolved: https://github.com/pytorch/pytorch/pull/148124
Approved by: https://github.com/albanD, https://github.com/zou3519, https://github.com/atalman

setup.py

@@ -1274,6 +1274,7 @@ def main():
         "include/c10/xpu/impl/*.h",
         "include/torch/*.h",
         "include/torch/csrc/*.h",
+        "include/torch/csrc/stable/*.h",
         "include/torch/csrc/api/include/torch/*.h",
         "include/torch/csrc/api/include/torch/data/*.h",
         "include/torch/csrc/api/include/torch/data/dataloader/*.h",