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/build.rs

@@ -0,0 +1,33 @@
+use std::env;
+use std::path::Path;
+use std::process::Command;
+
+// Mirror the per-crate convention (see xtrain-cuda/build.rs): with no nvcc/GPU
+// locally, emit `no_cuda` so the NCCL FFI + DDP code compiles (but is not linked
+// or run). On dash5, link NCCL exactly like xserv-distributed's build.rs.
+fn main() {
+    println!("cargo:rustc-check-cfg=cfg(no_cuda)");
+
+    let cuda_path = env::var("CUDA_HOME")
+        .or_else(|_| env::var("CUDA_PATH"))
+        .unwrap_or_else(|_| "/usr/local/cuda".to_string());
+
+    if !nvcc_available(&cuda_path) {
+        println!("cargo:warning=nvcc not found — skipping NCCL link (host-only build).");
+        println!("cargo:rustc-cfg=no_cuda");
+        return;
+    }
+
+    println!("cargo:rustc-link-search=native={cuda_path}/lib64");
+    // NCCL is installed as a system library on dash5.
+    println!("cargo:rustc-link-search=native=/usr/lib/x86_64-linux-gnu");
+    println!("cargo:rustc-link-lib=dylib=nccl");
+    println!("cargo:rustc-link-lib=dylib=cudart");
+}
+
+fn nvcc_available(cuda_path: &str) -> bool {
+    if Command::new("nvcc").arg("--version").output().is_ok() {
+        return true;
+    }
+    Path::new(&format!("{cuda_path}/bin/nvcc")).exists()
+}