Prioritize reentrant tasks and execute them recursively until close to limit
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/22397
Test Plan:
Added test for reentrant backwards with checkpoint and a test for a recursive backwards function (which should fail if we run all the reentrant tasks recursively in the same thread) and for testing priority of reentrant tasks.
~~Will add a test for priority of reentrant tasks in future pr.~~
Imported from OSS
Differential Revision: D16131955
fbshipit-source-id: 18301d45c1ec9fbeb566b1016dbaf7a84a09c7ac
