Add type hints for the _import_c_extension module (#51767)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51767
The `_import_c_extension.py` finds the right C extension library to use,
and then simply re-exports all of the symbols that it defines.
This adds a `_import_c_extension.pyi` file with type hints to let type
checkers like Pyre and Mypy know the names of the symbols that will be
re-exported from the C extension.
This does not define all of the symbols provided by the C extension,
but does define all of the symbols necessary to make type checkers happy
about other code in the `caffe2/python` directory.
ghstack-source-id: 121916324
Test Plan:
Was able to have Pyre successfully type check the `caffe2/python`
directory with this stub file plus a few other changes.
Confirmed that all of the dependent projects affected by this report no new
pyre issues in sandcastle.
Ran `python test/test_type_hints.py` in the PyTorch github repository and
confirmed it also passes.
Differential Revision: D26271726
Pulled By: simpkins
fbshipit-source-id: 6dbadcf02e0b2cc44a9e3cdabe9291c1250959b4
