[0.9.1][Dist][Bugfix] Fix mc2 process group to resolve self.cpu_group is None (#1831)

### What this PR does / why we need it?
This pr fixes the bug, which throw an error `self.cpu_group is None`.
This is mainly caused by the wrong group ranks of process groups
maintained in vllm-ascend. We need to take external dp size into account
to ensure it work fine with `external_launch` mode.

Related fixes: #1396
#1154

Signed-off-by: MengqingCao <cmq0113@163.com>

Author: MengqingCao

vllm_ascend/distributed/parallel_state.py

@@ -19,22 +19,22 @@ def model_parallel_initialized():
 
 def init_ascend_model_parallel(
     expert_parallel_size: int = 1,
-    world_size: Optional[int] = None,
     backend: Optional[str] = None,
 ):
     if model_parallel_initialized():
         return
     assert torch.distributed.is_initialized()
-    world_size = world_size or torch.distributed.get_world_size()
+    world_size = torch.distributed.get_world_size()
     backend = backend or torch.distributed.get_backend(
         get_world_group().device_group)
-    num_expert_parallel_groups = world_size // expert_parallel_size
 
+    # The layout of all ranks: ExternalDP * EP
+    # ExternalDP is the data parallel group that is not part of the model,
+    # every dp rank can generate independently (in verl integration).
+    all_ranks = torch.arange(world_size).reshape(-1, expert_parallel_size)
     global _MC2
-    group_ranks = []
-    for i in range(num_expert_parallel_groups):
-        ranks = list(range(i, world_size, num_expert_parallel_groups))
-        group_ranks.append(ranks)
+    group_ranks = all_ranks.unbind(0)
+    group_ranks = [x.tolist() for x in group_ranks]
 
     _MC2 = init_model_parallel_group(group_ranks,
                                      get_world_group().local_rank,

vllm_ascend/worker/worker.py

@@ -546,8 +546,7 @@ def _init_worker_distributed_environment(
         ensure_model_parallel_initialized(
             parallel_config.tensor_parallel_size,
             parallel_config.pipeline_parallel_size)
-        init_ascend_model_parallel(parallel_config.expert_parallel_size,
-                                   parallel_config.world_size_across_dp)
+        init_ascend_model_parallel(parallel_config.expert_parallel_size)
         ensure_kv_transfer_initialized(vllm_config)
 
 

vllm_ascend/worker/worker_v1.py

@@ -261,8 +261,7 @@ def _init_worker_distributed_environment(self) -> None:
         ensure_model_parallel_initialized(
             self.parallel_config.tensor_parallel_size,
             self.parallel_config.pipeline_parallel_size)
-        init_ascend_model_parallel(self.parallel_config.expert_parallel_size,
-                                   self.parallel_config.world_size_across_dp)
+        init_ascend_model_parallel(self.parallel_config.expert_parallel_size)
         ensure_kv_transfer_initialized(self.vllm_config)
 
     def _init_profiler(self):