Split out data_parallel tests from test_nn.py into a separate (#28297)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28297
Splitting data parallel tests out of test_nn.py since its easier to
manage and track these tests separately and failures can be routed to
appropriate POCs.
Test Plan: waitforbuildbot
Differential Revision: D18011663
fbshipit-source-id: 17ebf7c04e7dc7ff4c8d38458daab5b911bed75d
