use bitfield to shrink TensorImpl (#45263)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45263
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D23900587
Pulled By: bhosmer
fbshipit-source-id: 9214b887fde010bd7c8be848ee7846329c35752f
