[Runtime] Introduce MSCCLPP with NCCL equivalent interface (#16804)

* [Runtime] Introduce MSCCLPP with NCCL equivalent interface

* Add a fast and simple AllReduce kernel (sum only) using
  using mscclpp smChannel scratch for small reductions
  up to 2**24 bytes.

Author: csullivan

3rdparty/mscclpp/include/common.h

@@ -0,0 +1,107 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+#ifndef MSCCL_COMMON_HPP_
+#define MSCCL_COMMON_HPP_
+
+#if defined(__HIP_PLATFORM_AMD__)
+#define WARP_SIZE 64
+#define __syncwarp() __builtin_amdgcn_wave_barrier()
+#else
+#define WARP_SIZE 32
+#endif
+
+constexpr int NRANKS_PER_NODE = 8;
+constexpr int SCRATCH_SIZE = 1024 * 1024 * 70;  // 35 thread-blocks * 8 ranks * 256KB = 70MB
+
+template <typename To, typename From>
+__forceinline__ __device__ To bit_cast(const From& src) {
+  static_assert(sizeof(To) == sizeof(From), "Size mismatch for bit_cast");
+
+  union {
+    From f;
+    To t;
+  } u;
+  u.f = src;
+  return u.t;
+}
+
+template <typename T>
+__forceinline__ __device__ T add_elements(T a, T b) {
+  return a + b;
+}
+
+template <>
+__forceinline__ __device__ __half2 add_elements(__half2 a, __half2 b) {
+  return __hadd2(a, b);
+}
+
+template <typename T>
+__forceinline__ __device__ int4 add_vectors_helper(int4 a, int4 b) {
+  int4 ret;
+  ret.w = bit_cast<int, T>(add_elements(bit_cast<T, int>(a.w), bit_cast<T, int>(b.w)));
+  ret.x = bit_cast<int, T>(add_elements(bit_cast<T, int>(a.x), bit_cast<T, int>(b.x)));
+  ret.y = bit_cast<int, T>(add_elements(bit_cast<T, int>(a.y), bit_cast<T, int>(b.y)));
+  ret.z = bit_cast<int, T>(add_elements(bit_cast<T, int>(a.z), bit_cast<T, int>(b.z)));
+  return ret;
+}
+
+template <typename T>
+__forceinline__ __device__ int4 add_vectors(int4 a, int4 b) {
+  return add_vectors_helper<T>(a, b);
+}
+
+template <>
+__forceinline__ __device__ int4 add_vectors<__half>(int4 a, int4 b) {
+  return add_vectors_helper<__half2>(a, b);
+}
+
+template <typename T>
+__forceinline__ __device__ uint2 add_vectors_helper(uint2 a, uint2 b) {
+  uint2 ret;
+  ret.x = bit_cast<int, T>(add_elements(bit_cast<T, int>(a.x), bit_cast<T, int>(b.x)));
+  ret.y = bit_cast<int, T>(add_elements(bit_cast<T, int>(a.y), bit_cast<T, int>(b.y)));
+  return ret;
+}
+
+template <typename T>
+__forceinline__ __device__ uint2 add_vectors(uint2 a, uint2 b) {
+  return add_vectors_helper<T>(a, b);
+}
+
+template <>
+__forceinline__ __device__ uint2 add_vectors<__half>(uint2 a, uint2 b) {
+  return add_vectors_helper<__half2>(a, b);
+}
+
+template <typename T>
+__forceinline__ __device__ int add_vectors_helper(int a, int b) {
+  return bit_cast<int, T>(add_elements(bit_cast<T, int>(a), bit_cast<T, int>(b)));
+}
+
+template <typename T>
+__forceinline__ __device__ int add_vectors(int a, int b) {
+  return add_vectors_helper<T>(a, b);
+}
+
+template <>
+__forceinline__ __device__ int add_vectors<__half>(int a, int b) {
+  return add_vectors_helper<__half2>(a, b);
+}
+
+template <typename T>
+__forceinline__ __device__ uint32_t add_vectors_helper(uint32_t a, uint32_t b) {
+  return bit_cast<uint32_t, T>(add_elements(bit_cast<T, uint32_t>(a), bit_cast<T, uint32_t>(b)));
+}
+
+template <typename T>
+__forceinline__ __device__ uint32_t add_vectors(uint32_t a, uint32_t b) {
+  return add_vectors_helper<T>(a, b);
+}
+
+template <>
+__forceinline__ __device__ uint32_t add_vectors<__half>(uint32_t a, uint32_t b) {
+  return add_vectors_helper<__half2>(a, b);
+}
+
+#endif  // MSCCL_COMMON_HPP_