handle scenario when GPU support is not available and p2p_access_pattern is empty...

handle scenario when GPU support is not available and p2p_access_pattern is empty (#17974)

Summary:
Observed that when there is no GPU support available `workspace `sets `GetGpuPeerAccessPattern `to `[]` in
https://github.com/pytorch/pytorch/blob/master/caffe2/python/workspace.py#L79
and this case is not handled in https://github.com/pytorch/pytorch/blob/master/caffe2/python/data_parallel_model.py#L1065.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17974

Differential Revision: D14517066

Pulled By: ezyang

fbshipit-source-id: 186911d95c07e9a55ab82a41d0c7c919e4281bb4