Add generate_vbe_metadata CPU fallback (#3183)

Summary:
X-link: facebookresearch/FBGEMM#279

Pull Request resolved: #3183

Add generate_vbe_metadata for CPU

Reviewed By: q10

Differential Revision: D63494418

fbshipit-source-id: 3936e2546ccf4fb89632df5c49148141adbabe71

fbgemm_gpu/include/fbgemm_gpu/split_embeddings_utils.cuh

@@ -12,14 +12,7 @@
 #include <cuda.h>
 #include <cuda_runtime.h>
 #include "fbgemm_gpu/embedding_common.h"
-
-// These values are adjusted in backward based on B and T
-constexpr int DEFAULT_INFO_NUM_BITS = 32;
-constexpr int DEFAULT_INFO_B_NUM_BITS = 26;
-constexpr uint32_t DEFAULT_INFO_B_MASK = (1u << DEFAULT_INFO_B_NUM_BITS) - 1;
-constexpr uint32_t MAX_T =
-    (1u << (DEFAULT_INFO_NUM_BITS - DEFAULT_INFO_B_NUM_BITS)) - 1;
-constexpr uint32_t MAX_B = (1u << DEFAULT_INFO_B_NUM_BITS) - 1;
+#include "fbgemm_gpu/split_embeddings_utils.h"
 
 /**
  * "Transpose" embedding inputs by sorting indices by their values.
@@ -50,11 +43,6 @@ transpose_embedding_input(
     const int64_t fixed_L_per_warp = 0,
     const int64_t num_warps_per_feature = 0);
 
-std::tuple<int64_t, int64_t>
-get_infos_metadata(at::Tensor unused, int64_t B, int64_t T);
-
-std::tuple<int32_t, uint32_t> adjust_info_B_num_bits(int32_t B, int32_t T);
-
 // Use these functions instead of directly calling cub functions
 // to reduce code size and compilation time.
 // Arguments are the same as cub::DeviceRadixSort::SortPairs
@@ -77,15 +65,3 @@ DECL_RADIX_SORT_PAIRS_FN(int64_t, int64_t);
 DECL_RADIX_SORT_PAIRS_FN(int64_t, int32_t);
 
 #undef DECL_RADIX_SORT_PAIRS_FN
-
-std::tuple<at::Tensor /*row_output_offsets*/, at::Tensor /*b_t_map*/>
-generate_vbe_metadata(
-    const at::Tensor& B_offsets,
-    const at::Tensor& B_offsets_rank_per_feature,
-    const at::Tensor& output_offsets_feature_rank,
-    const at::Tensor& D_offsets,
-    const int64_t D,
-    const bool nobag,
-    const int64_t max_B_feature_rank,
-    const int64_t info_B_num_bits,
-    const int64_t total_B);

fbgemm_gpu/include/fbgemm_gpu/split_embeddings_utils.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <ATen/ATen.h>
+
+// These values are adjusted in backward based on B and T
+constexpr int DEFAULT_INFO_NUM_BITS = 32;
+constexpr int DEFAULT_INFO_B_NUM_BITS = 26;
+constexpr uint32_t DEFAULT_INFO_B_MASK = (1u << DEFAULT_INFO_B_NUM_BITS) - 1;
+constexpr uint32_t MAX_T =
+    (1u << (DEFAULT_INFO_NUM_BITS - DEFAULT_INFO_B_NUM_BITS)) - 1;
+constexpr uint32_t MAX_B = (1u << DEFAULT_INFO_B_NUM_BITS) - 1;
+
+std::tuple<int64_t, int64_t>
+get_infos_metadata(at::Tensor unused, int64_t B, int64_t T);
+
+std::tuple<int32_t, uint32_t> adjust_info_B_num_bits(int32_t B, int32_t T);
+
+std::tuple<at::Tensor /*row_output_offsets*/, at::Tensor /*b_t_map*/>
+generate_vbe_metadata(
+    const at::Tensor& B_offsets,
+    const at::Tensor& B_offsets_rank_per_feature,
+    const at::Tensor& output_offsets_feature_rank,
+    const at::Tensor& D_offsets,
+    const int64_t D,
+    const bool nobag,
+    const int64_t max_B_feature_rank,
+    const int64_t info_B_num_bits,
+    const int64_t total_B);

fbgemm_gpu/src/split_embeddings_utils/split_embeddings_utils.cpp

@@ -33,6 +33,22 @@ generate_vbe_metadata_meta(
   return {row_output_offsets, b_t_map};
 }
 
+std::tuple<Tensor /*row_output_offsets*/, Tensor /*b_t_map*/>
+generate_vbe_metadata_cpu(
+    const Tensor& B_offsets,
+    const Tensor& B_offsets_rank_per_feature,
+    const Tensor& output_offsets_feature_rank,
+    const Tensor& D_offsets,
+    const int64_t D,
+    const bool nobag,
+    const c10::SymInt max_B_feature_rank,
+    const int64_t info_B_num_bits,
+    const c10::SymInt total_B) {
+  Tensor row_output_offsets = output_offsets_feature_rank;
+  Tensor b_t_map = B_offsets_rank_per_feature;
+  return {row_output_offsets, b_t_map};
+}
+
 } // namespace
 
 TORCH_LIBRARY_FRAGMENT(fbgemm, m) {
@@ -68,6 +84,7 @@ TORCH_LIBRARY_FRAGMENT(fbgemm, m) {
   DISPATCH_TO_CUDA("transpose_embedding_input", transpose_embedding_input);
   DISPATCH_TO_CUDA("get_infos_metadata", get_infos_metadata);
   DISPATCH_TO_CUDA("generate_vbe_metadata", generate_vbe_metadata);
+  DISPATCH_TO_CPU("generate_vbe_metadata", generate_vbe_metadata_cpu);
 }
 
 TORCH_LIBRARY_IMPL(fbgemm, Meta, m) {