[inductor][easy] Enable Mypy Checking in torch/_inductor/kernel/ (#108678)
Summary: Looks like these already pass (and torch/_inductor/kernel/mm_plus_mm_new.py does not exist)
Test Plan: `lintrunner torch/_inductor/kernel/mm.py torch/_inductor/kernel/bmm.py torch/_inductor/kernel/__init__.py torch/_inductor/kernel/mm_plus_mm.py`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108678
Approved by: https://github.com/eellison
