Refactor / Unify event loop across PD-Disagg, Overlap, DP-Attn cases (#12839)

Co-authored-by: cctry <17473714+cctry@users.noreply.github.com>

python/sglang/srt/two_batch_overlap.py

@@ -68,7 +68,7 @@ def get_token_num_per_seq(
 
 # TODO: may smartly disable TBO when batch size is too small b/c it will slow down
 def compute_split_seq_index(
-    forward_mode: "ForwardMode",
+    forward_mode: ForwardMode,
     num_tokens: int,
     extend_lens: Optional[Sequence[int]],
     token_num_per_seq: Optional[int],
@@ -79,7 +79,7 @@ def compute_split_seq_index(
     elif forward_mode.is_target_verify() or forward_mode.is_decode():
         assert token_num_per_seq is not None
         return (num_tokens // token_num_per_seq) // 2
-    elif forward_mode.is_idle():
+    elif forward_mode.is_idle() or forward_mode.is_prebuilt_extend():
         assert num_tokens == 0
         return 0
     else:
@@ -381,6 +381,8 @@ class TboDPAttentionPreparer:
                 or local_batch.forward_mode.is_decode()
             ):
                 num_tokens = local_batch.batch_size() * token_num_per_seq
+            elif local_batch.forward_mode.is_prebuilt_extend():
+                num_tokens = 0
             else:
                 num_tokens = local_batch.extend_num_tokens
             self.local_tbo_split_seq_index = compute_split_seq_index(