Fix potential random layout inconsistency issues in sparse attention modules (#534)

* 1) Register layout as buffer of module so that we can save/load checkpoint; 2) Add a broadcast of layout at the beginning to ensure different processes will have consistent layout during distributed training.

* Add docstring for max_seq_length argument in SparseSelfAttention
Co-authored-by: NZhun Liu <zhunliu@microsoft.com>
Co-authored-by: NJeff Rasley <jerasley@microsoft.com>

deepspeed/ops/sparse_attention/sparse_self_attention.py

@@ -5,6 +5,7 @@ Copyright 2020 The Microsoft DeepSpeed Team
 import torch.nn as nn
 from torch.nn.functional import *
 import torch
+from torch import distributed as dist
 from collections import namedtuple
 from deepspeed.ops.sparse_attention import MatMul, Softmax, SparsityConfig
 import sys
@@ -22,29 +23,50 @@ class SparseSelfAttention(nn.Module):
         # SparsityConfig parameters needs to be set accordingly
         sparsity_config=SparsityConfig(num_heads=4),
         key_padding_mask_mode='add',
-        attn_mask_mode='mul'):
+        attn_mask_mode='mul',
+        max_seq_length=2048):
         """Initialize the sparse self attention layer.
         Arguments:
             sparsity_config: optional: this parameter determins sparsity pattern configuration; it is based on SparsityConfig class.
             key_padding_mask_mode: optional: a string determining if key padding mask needs to be added, `add`, or be multiplied, `mul`.
             attn_mask_mode: optional: a string determining if attention mask needs to be added, `add`, or be multiplied, `mul`.
+            max_seq_length: optional: the maximum sequence length this sparse attention module will be applied to; it controls the size of the master_layout.
         """
         super().__init__()
 
         # sparsity information
         self.sparsity_config = sparsity_config
 
+        # initialize sparse layout and register as buffer
+        master_layout = self.sparsity_config.make_layout(max_seq_length)
+        self.register_buffer("master_layout", master_layout)
+        self._need_layout_synchronization = True
+
         # mask modes
         self.key_padding_mask_mode = key_padding_mask_mode
         self.attn_mask_mode = attn_mask_mode
 
     ops = dict()
 
+    def get_layout(self, L):
+        # if layout is never synchronized across GPUs, broadcast the layout from global rank 0
+        if self._need_layout_synchronization and dist.is_initialized():
+            dist.broadcast(self.master_layout, src=0)
+            self._need_layout_synchronization = False
+
+        if (L % self.sparsity_config.block != 0):
+            raise ValueError(
+                f'Sequence Length, {L}, needs to be dividable by Block size {self.sparsity_config.block}!'
+            )
+
+        num_blocks = L // self.sparsity_config.block
+        return self.master_layout[..., :num_blocks, :num_blocks].cpu()  # layout needs to be a CPU tensor
+
     # add to cache
     def get_ops(self, H, L):
         import sys
         if L not in SparseSelfAttention.ops:
-            sparsity_layout = self.sparsity_config.make_layout(L)
+            sparsity_layout = self.get_layout(L)
             sparse_dot_sdd_nt = MatMul(sparsity_layout,
                                        self.sparsity_config.block,
                                        'sdd',