Fix DistributedDataParallelTest.test_accumulate_gradients (#20351)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20351
This was broken because of a merge race between #20282 and the stack in #20236.
Cleaned up the test and comments a bit as well.
Differential Revision: D15292786
fbshipit-source-id: a4379ea700cad959d3a6921fc5ddf9384fb8f228
