.github: Add workflow to build libtorch (#53292)

Summary:
Based on https://github.com/pytorch/pytorch/issues/50633 and https://github.com/pytorch/pytorch/issues/51243.

Things left to do:

- [x] modify `.github/scripts/generate_binary_build_matrix.py` further
  - [x] add option for not iterating over Python version
  - [x] add `LIBTORCH_VARIANT`
  - [x] add option for cxx11
  - [x] fix the artifact uploading
  - [x] remove `pull_request` hook before merging

Pull Request resolved: https://github.com/pytorch/pytorch/pull/53292

Test Plan: [CI](https://github.com/pytorch/pytorch/actions/runs/665781075).

Reviewed By: seemethere

Differential Revision: D27189150

Pulled By: samestep

fbshipit-source-id: ec91e1f0b75b8c93613d55801585ed975697be03

mypy-strict.ini

@@ -35,6 +35,7 @@ implicit_reexport = False
 strict_equality = True
 
 files =
+    .github/scripts/generate_binary_build_matrix.py,
     tools/autograd/*.py,
     tools/codegen/gen.py,
     tools/mypy_wrapper.py,