Rename createLevelsBitset -> createVmapLevelsBitset; move it (#46190)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46190
Moved `createLevelsBitset` to BatchedTensorImpl.h and renamed it to
`createVmapLevelsBitset`. I moved it there because I want to use it in
another file in a future diff.
Test Plan: Imported from OSS
Reviewed By: malfet
Differential Revision: D24335241
Pulled By: zou3519
fbshipit-source-id: 1c225a00b6da9c69dc7fbc61516bf7257298355c
