dist: nccl ffi + comm bootstrap

New crate xtrain-distributed (mirrors xserv-distributed): hand-written NCCL
FFI (GetUniqueId / CommInitRank / AllReduce / CommDestroy / Group{Start,End},
ncclUniqueId passed by value per the NCCL ABI) and a safe DdpContext wrapper —
rank 0 mints the UniqueId, every rank inits its communicator under a group, and
all_reduce_average_grads in-place AllReduce(sum)s each param's .grad() device
buffer then scales by 1/world (reuses T7's scale_inplace kernel). AllReduce runs
on the null stream so it orders with the model's kernels (no extra barrier).

build.rs follows the per-crate convention: no nvcc -> no_cuda cfg (crate
compiles to empty, cargo check passes host-side); with nvcc, links -lnccl
-lcudart like xserv-distributed's build.rs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xtrain-distributed/src/ffi.rs

@@ -0,0 +1,76 @@
+//! Minimal NCCL FFI bindings (hand-written, like the CUDA bindings in
+//! xtrain-cuda). Only the collectives data-parallel training needs:
+//! unique-id creation, communicator init/destroy, and AllReduce. Mirrors
+//! xserv-distributed's FFI.
+
+use std::ffi::c_void;
+use std::os::raw::c_char;
+use xtrain_cuda::ffi::CudaStream;
+
+/// Opaque NCCL communicator handle (`ncclComm_t`).
+pub type NcclComm = *mut c_void;
+
+/// `ncclUniqueId` is a 128-byte opaque blob shared from rank 0 to every rank.
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub struct NcclUniqueId {
+    pub internal: [c_char; 128],
+}
+
+impl Default for NcclUniqueId {
+    fn default() -> Self {
+        Self { internal: [0; 128] }
+    }
+}
+
+// ncclDataType_t (subset) — DDP all-reduces fp32 gradients.
+pub const NCCL_FLOAT32: i32 = 7;
+
+// ncclRedOp_t
+pub const NCCL_SUM: i32 = 0;
+
+// ncclResult_t
+pub const NCCL_SUCCESS: i32 = 0;
+
+unsafe extern "C" {
+    pub fn ncclGetUniqueId(uid: *mut NcclUniqueId) -> i32;
+    // ncclUniqueId is passed BY VALUE (a 128-byte struct) per the NCCL ABI.
+    pub fn ncclCommInitRank(
+        comm: *mut NcclComm,
+        nranks: i32,
+        commid: NcclUniqueId,
+        rank: i32,
+    ) -> i32;
+    pub fn ncclCommDestroy(comm: NcclComm) -> i32;
+    pub fn ncclAllReduce(
+        sendbuff: *const c_void,
+        recvbuff: *mut c_void,
+        count: usize,
+        datatype: i32,
+        op: i32,
+        comm: NcclComm,
+        stream: CudaStream,
+    ) -> i32;
+    pub fn ncclGroupStart() -> i32;
+    pub fn ncclGroupEnd() -> i32;
+    pub fn ncclGetErrorString(result: i32) -> *const c_char;
+}
+
+pub fn err_string(result: i32) -> String {
+    unsafe {
+        let p = ncclGetErrorString(result);
+        if p.is_null() {
+            return format!("nccl error {result}");
+        }
+        std::ffi::CStr::from_ptr(p).to_string_lossy().into_owned()
+    }
+}
+
+pub fn check(result: i32, what: &str) {
+    assert_eq!(
+        result,
+        NCCL_SUCCESS,
+        "{what} failed: {}",
+        err_string(result)
+    );
+}