[inductor][easy] Enable Mypy Checking for torch/_inductor/decomposition.py (#108682)
Summary: Looks like one simple type mismatch between `get_decompositions()` and `remove_decompositions()`
Test Plan: `lintrunner torch/_inductor/decomposition.py`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108682
Approved by: https://github.com/eellison
