distributed: NCCL tensor-parallel primitives (TpContext + AllReduce)

New xserv-distributed crate: hand-written NCCL FFI, TpContext (one rank per
thread, bound to one GPU), and in-place BF16 AllReduce on the null stream so
it orders naturally with the model's kernels. 2-GPU AllReduce test included.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/xserv-distributed/tests/allreduce.rs

@@ -0,0 +1,50 @@
+//! 2-GPU AllReduce smoke test. Skips if fewer than 2 GPUs are present.
+
+use half::bf16;
+use std::thread;
+use xserv_cuda::{device, GpuBuffer};
+use xserv_distributed::{get_unique_id, TpContext};
+
+#[test]
+fn allreduce_two_gpu_sum() {
+    let world = 2usize;
+    if device::device_count().unwrap_or(0) < world as i32 {
+        eprintln!("skip: need >= {world} GPUs");
+        return;
+    }
+
+    let id = get_unique_id();
+    let n = 4096usize;
+
+    let handles: Vec<_> = (0..world)
+        .map(|rank| {
+            let id = id;
+            thread::spawn(move || {
+                let ctx = TpContext::init(rank, world, id, rank as u32);
+
+                // Rank r fills its buffer with (r + 1).
+                let val = bf16::from_f32((rank + 1) as f32);
+                let host = vec![val; n];
+                let src = unsafe {
+                    std::slice::from_raw_parts(host.as_ptr() as *const u8, n * 2)
+                };
+                let mut buf = GpuBuffer::alloc(n * 2).unwrap();
+                buf.copy_from_host(src).unwrap();
+
+                ctx.all_reduce_sum_bf16(&mut buf, n);
+
+                let mut out = vec![0u8; n * 2];
+                buf.copy_to_host(&mut out).unwrap();
+                let res = unsafe { std::slice::from_raw_parts(out.as_ptr() as *const bf16, n) };
+                (res[0].to_f32(), res[n - 1].to_f32())
+            })
+        })
+        .collect();
+
+    // sum over ranks of (r+1) = 1 + 2 = 3
+    for h in handles {
+        let (first, last) = h.join().unwrap();
+        assert_eq!(first, 3.0, "AllReduce(sum) first element");
+        assert_eq!(last, 3.0, "AllReduce(sum) last element");
+    }
+}