Deduplicate instances caching allocator, so that we only have one instance. (#16720)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16720
I'm taking the deduplication slowly because there is something here
that is causing problems, and I want to figure out what it is.
Reviewed By: dzhulgakov
Differential Revision: D13943194
fbshipit-source-id: cbc08fee5862fdcb393b9dd5b1d2ac7250f77c4b
