ciflow/trunk/186173: [pipelining] Optional per-direction P2P communicators for PipelineStage

Summary:
Pipeline parallelism issues all send/recv on a single communicator.

NCCL-group coalescing (batch_isend_irecv -> one ncclGroupStart/End) makes a single mixed
send+recv batch deadlock-free, but it does not remove ordering hazards across
separate batches.

This adds per-direction PP communicators. When enabled, forward (
r->r+1) and backward (r->r-1) P2P are carried on two separate
communicators derived from the stage's group via split_group.

Test Plan:

• test_pp_per_direction_pipeline_stage.py
• test_pp_torchcomms_p2p_deadlock.py