Enable mypy check in torch/_inductor/fx_passes/post_grad.py (#106839)
Fixes #105230
```shell
$ lintrunner -a torch/_inductor/fx_passes/post_grad.py
FLAKE8 success!
CLANGFORMAT success!
MYPYNOFOLLOW success!
MYPY success!
MYPYSTRICT success!
CLANGTIDY success!
TYPEIGNORE success!
NOQA success!
CIRCLECI success!
SPACES success!
NEWLINE success!
CONSTEXPR success!
NATIVEFUNCTIONS success!
INCLUDE success!
TABS success!
PYBIND11_INCLUDE success!
ERROR_PRONE_ISINSTANCE success!
PYBIND11_SPECIALIZATION success!
PYPIDEP success!
RAWCUDA success!
CUBINCLUDE success!
EXEC success!
RAWCUDADEVICE success!
ROOT_LOGGING success!
DEPLOY_DETECTION success!
ACTIONLINT success!
CALL_ONCE success!
TESTOWNERS success!
WORKFLOWSYNC success!
CMAKE success!
COPYRIGHT success!
BAZEL_LINTER success!
SHELLCHECK success!
LINTRUNNER_VERSION success!
UFMT success!
ONCE_FLAG success!
RUFF success!
ok No lint issues.
Successfully applied all patches.
```
```shell
$ mypy torch/_inductor/fx_passes/post_grad.py
Success: no issues found in 1 source file
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106839
Approved by: https://github.com/Skylion007
