Enable TensorPipe's SHM transport (#50760)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50760
The SHM transport uses shared-memory-backed ringbuffers to transfer small payloads between processes on the same machine.
It was disabled in v1.6 due to a CMake mishap but we've since realized that it also doesn't work that well in docker and other setups. Enabling it here to see whether CircleCI fails.
ghstack-source-id: 120470890
Test Plan: Exported three times to CircleCI with tests consistently passing
Reviewed By: mrshenli
Differential Revision: D23814828
fbshipit-source-id: f355cb6515776debad536924de4f4d3fbb05a874
