Remove NodeExecStats and AllocatorMemoryUsed (#21419)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21419
Removed ```node_stats``` and unused imports
Reviewed By: orionr
Differential Revision: D15672824
fbshipit-source-id: 6167c80c081d925f02a1d279f3af0e1b8de66752
