Enable mypy check in torch/_inductor/kernel/unpack_mixed_mm.py (#107445)
Fixes #105230
```shell
$ lintrunner init && lintrunner -a torch/_inductor/kernel/unpack_mixed_mm.py
...
ok No lint issues.
Successfully applied all patches.
```
```shell
$ mypy torch/_inductor/kernel/unpack_mixed_mm.py
Success: no issues found in 1 source file
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107445
Approved by: https://github.com/ezyang
