Back out "Faster gc_count update for CUDACachingAllocator" (#108632)
Summary:
Original commit changeset: 1d04ae368fd8
Original Phabricator Diff: D48481557
block.pool is not guaranteed to be not nullptr
Test Plan: CI
Differential Revision: D49003756
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108632
Approved by: https://github.com/houseroad
