add .cache to gitignore (#45017)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45017
this is the default indexing folder for clangd 11.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D23817619
Pulled By: suo
fbshipit-source-id: 6a60136e591b2fec3d432ac5343cb76ac0934502
