[caffe2] cache NextName indexes for faster name generation (#47768)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47768
This stores the next ID for a given NextName(prefix, output_id) so repeated calls to NextName are significantly faster. This accounts for ~65% of time spent for large models.
Test Plan:
buck test //caffe2/caffe2/python/...
will launch canary job before landing to ensure no regressions + confirm speedup
Reviewed By: dzhulgakov
Differential Revision: D24876961
fbshipit-source-id: 668d73060d800513bc72d7cd405a47d15c4acc34
