Use batched P2P to enhance alltoall small message performance (#1902)

* Batch P2P operations (2 per CU/channel) and update channel-part mapping

- Revert bitreversal and fix channel mapping to be compatible with P2P batching and avoid hangs

- P2P batching is only used for more than 2 nodes to avoid aggregating intra-node traffic when it is dominant for less than 2 nodes

* Address single node regression and channel per net peer

* Add batching threshold

* Add enable switch for batching

* Update CHANGELOG.md

* Add minor comment change

* Update CHANGELOG.md

Co-authored-by: Jeffrey Novotny <jnovotny@amd.com>

* Update CHANGELOG.md

Co-authored-by: Jeffrey Novotny <jnovotny@amd.com>

[ROCm/rccl commit: c1e1f2faeb]

projects/rccl/src/device/sendrecv.h

@@ -22,7 +22,7 @@ struct RunWorkBatch<ncclFuncSendRecv, T, RedOp, NCCL_ALGO_RING, NCCL_PROTO_SIMPL
     bool useLargeChunk = (work->sendIpcReg && ncclShmem.comm.isAllNvlink) || work->sendNetReg;
     int chunkSize = useLargeChunk ? NCCL_MAX_NET_SIZE : u32fp8Decode(work->sendChunkSize_u32fp8);
     int stepSize = useLargeChunk ? NCCL_MAX_NET_SIZE : ncclShmem.comm.p2pChunkSize;
-  
+
 #if defined(ENABLE_NPKIT)
     bool isNpKitThread = (tid == 0);
     int npKitCtxIdx = blockIdx.x * NCCL_MAX_DEV_WORK_P2P_ELEMENTS + group;