[v1.6] [RPC docs] Remove mention of TensorPipe's SHM and CMA backends as they're not built (#41229)
Summary:
In short, we messed up. The SHM and CMA backends of TensorPipe are Linux-specific and thus they are guarded by a #ifdef in the agent's code. Due to a mishap with CMake (due the fact that TensorPipe has two CMake files, one for PyTorch and a "standalone" one) we were not correctly propagating some flags and these #ifdefs were always false. This means that these two backends have always been disabled and have thus never been covered by our OSS CI. It would be irresponsible to enable them now in v1.6, so instead we remove any mention of them from the docs.
Note that this is perhaps not as bad as it sounds. These two backends were providing higher performance (latency) when the two endpoints were on the same machine. However, I suspect that most RPC users will only do transfers across machines, for which SHM and CMA wouldn't have played any role.
Original PR against master: #41200 (merged as dde3d5f4a8f713ecc4649d776565b68ca75ae5c8)
Test Plan: Docs only
