Fix BN tests for >= 8 GPU test environments (#19049)
Summary:
DDP does not support replicating BN layers within a process. Existing BN tests fail if the test environment has more than 8 GPUs. This is fixed by explicitly setting each process to use a single replica.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19049
Differential Revision: D14845286
Pulled By: mrshenli
fbshipit-source-id: 937dda5081d415ece48b21f2781b6b4e008dd42f
