Relax set_num_threads restriction in parallel native case (#27947)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27947
Don't throw exception if the requested size is the same as the currently
used one
Test Plan:
ATEN_THREADING=NATIVE python setup.py develop --cmake
Imported from OSS
Differential Revision: D17919416
fbshipit-source-id: 411f7c9bd6a46e7a003b43a200c2ce3b76453a2e
