Free all blocks with outstanding events on OOM-retry (#19222)
Summary:
The caching allocator tries to free all blocks on an out-of-memory
error. Previously, it did not free blocks that still had outstanding
stream uses. This change synchronizes on the outstanding events and
frees those blocks.
See #19219
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19222
Differential Revision: D14925071
Pulled By: colesbury
fbshipit-source-id: a2e9fe957ec11b00ea8e6c0468436c519667c558
