Enable mypy check in torch/_inductor/fx_passes/post_grad.py (#107449)
Fixes #105230
```shell
$ lintrunner init && lintrunner -a torch/_inductor/fx_passes/post_grad.py
...
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/107449
Approved by: https://github.com/ezyang
