fixing the bug of flash_attn import and the wrong gather index when u…

…sing flash_attn_cuda in sequence parallel (#406)

Co-authored-by: Jinghan Yao <jyao@orthus.nic.uoregon.edu>

megatron/model/transformer.py

@@ -37,9 +37,12 @@
 try:
     # FlashAttention (1.x)
     from flash_attn.flash_attn_interface import flash_attn_unpadded_func
-    from flash_attn.flash_attn_triton import flash_attn_func
 except ImportError:
     flash_attn_unpadded_func = None
+
+try:
+    from flash_attn.flash_attn_triton import flash_attn_func
+except ImportError:
     flash_attn_func = None
 
 try:
@@ -599,7 +602,11 @@ def __init__(self, config, layer_number,
         if self.enable_ds_sequence_parallel:
             assert dist_attn_supported, 'Distributed attention is not supported in this DeepSpeed version'
             assert args.num_attention_heads % parallel_state.get_sequence_parallel_world_size() == 0
-            self.dist_attn = DistributedAttention(local_attn, parallel_state.get_sequence_parallel_group())
+            self.dist_attn = DistributedAttention(
+                local_attn, 
+                parallel_state.get_sequence_parallel_group(), 
+                gather_idx=1 if args.use_flash_attn_v1 or args.use_flash_attn_v2 else 0) 
+            # flash_attn_cuda assumes [b, s, nh, hd] layout, we need to make sure all2all gathers into the correct sequence dimension.
         else:
             if self.use_flash_attn:
                 self.core_attention_flash = local_attn