Use reduce scatter for DP (#8539)

python/sglang/srt/model_executor/forward_batch_info.py

@@ -628,8 +628,10 @@ class ForwardBatch:
         self.dp_padding_mode = dp_padding_mode
 
         if dp_padding_mode.is_max_len():
-            # when DP gather mode is all gather, we will use all_gather_into_tensor to gather hidden states,
-            # where transferred tokens should be padded to the same length.
+            # when DP gather mode is all gather, we will use
+            # all_gather_into_tensor to gather hidden states, where transferred
+            # tokens should be padded to the same length. We will also use
+            # reduce-scatter instead of all-reduce after MLP.
             max_num_tokens = max(global_num_tokens)
             global_num_tokens = [max_num_tokens] * sync_group_size
             buffer_len = max_num_tokens * sync_group_size