Do layernorm before allgather for DP attention (#8631)

Author: trevor-m

python/sglang/srt/layers/communicator.py

@@ -404,14 +404,24 @@ def _gather_hidden_states_and_residual(
         if context.attn_dp_size != 1:
             if context.attn_tp_rank == 0:
                 hidden_states += residual
+
+            # Perform layernorm on smaller data before comm. Only valid when attn_tp_size is 1 (tp_size == dp_size)
+            use_layer_norm_before_gather = context.attn_tp_size == 1
+            if use_layer_norm_before_gather:
+                residual.copy_(hidden_states)
+                if hidden_states.shape[0] != 0:
+                    hidden_states = layernorm(hidden_states)
+
             hidden_states, local_hidden_states = (
                 forward_batch.gathered_buffer,
                 hidden_states,
             )
             dp_gather_partial(hidden_states, local_hidden_states, forward_batch)
-            dp_scatter(residual, hidden_states, forward_batch)
-            if hidden_states.shape[0] != 0:
-                hidden_states = layernorm(hidden_states)
+
+            if not use_layer_norm_before_gather:
+                dp_scatter(residual, hidden_states, forward_batch)
+                if hidden_states.shape[0] != 0:
+                    hidden_states = layernorm(hidden_states)
         else:
             # According to the discussion in https://github.com/flashinfer-ai/flashinfer/issues/1223#issuecomment-3047256465
             # We set the max token num to 128 for allreduce fusion with min-latency case(use_oneshot=True).