Allow ReadyQueue to handle empty tasks (#15791)
Summary:
Allow the comparison function used in ReadyQueue to handle the empty FunctionTasks created by the reentrant autograd.
Fix #11732
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15791
Differential Revision: D13598006
Pulled By: soumith
fbshipit-source-id: 0bfdf28a735fbfe44f0fdbaf8b74a6198e6a1984
