Add CTRAN dispatch and arg validation to ncclAlltoAll

[YulunW]

Summary:
NCCLX ships two all-to-all entry points with identical signatures but different
implementations, sitting ~230 lines apart in the same file:

• ncclAlltoAll (lowercase t, collectives.cc:117) — the upstream NVIDIA API,
  added in NCCL 2.28. Dispatches straight to ncclEnqueueCheck(ncclFuncAlltoAll).
  No CTRAN, no argument validation.
• ncclAllToAll (capital T, collectives.cc:346) — Meta-created before upstream
  had an all-to-all. Has the CTRAN switch plus argument checks, and falls back to a
  hand-rolled grouped baseSend/baseRecv loop.

This is the first diff in a stack that de-dups the two down to ncclAlltoAll. It
brings ncclAlltoAll up to parity so it is a drop-in replacement, without touching
ncclAllToAll yet — both entry points remain fully functional after this diff, so
nothing is broken at any point in the stack.

Grafted onto ncclAlltoAll, mirroring how ncclAllGather (collectives.cc:93)
already layers CTRAN on top of the upstream baseline:

• count == 0 early-out
• SetCudaDevRAII
• CudaPtrCheck on sendbuff and recvbuff
• in-place (sendbuff == recvbuff) rejection with ncclInvalidArgument
• the NCCL_ALLTOALL_ALGO / ctranAllToAllSupport / ctranAllToAll dispatch

The non-CTRAN fallback stays on the upstream ncclEnqueueCheck(ncclFuncAlltoAll)
path rather than adopting the Meta send/recv loop. These are closer than they look:
taskAppend (enqueue.cc:3044-3051) already decomposes ncclFuncAlltoAll into the
same 2 * nRanks P2P send/recv tasks the Meta loop builds by hand — it never becomes
a ncclTaskColl and never reaches a device kernel. Staying on the upstream path also
propagates collAPI = ncclFuncAlltoAll to the net plugin (already whitelisted at
net.cc:218), emits one AlltoAll NVTX range instead of 2N Send/Recv ranges,
and keeps divergence from upstream small for future rebases.

Applied identically to v2_29 and v2_30; the two files are byte-identical here.

Differential Revision: D115510404

[meta-codesync]

@YulunW has exported this pull request. If you are a Meta employee, you can view the originating Diff in D115510404.