[main][BugFix] Avoided a bug of torch_npu.npu_mm_reduce_scatter_base when sp size >= 16 (#6168)

### What this PR does / why we need it?
If `sp` is enabled and `tp_size` >= 16,
`torch_npu.npu_mm_reduce_scatter_base` will raises a exception.
After consulting with the operator developer, we learned that the
operator does not work when `tp` = 16.
So, we disable the operator when `tp` = 16.

### Does this PR introduce _any_ user-facing change?
N/A

### How was this patch tested

We started a server with `sp` enabled and `tp` = 16.

It started successfully.

```text
(APIServer pid=1855938) INFO:     Started server process [1855938]
(APIServer pid=1855938) INFO:     Waiting for application startup.
(APIServer pid=1855938) INFO:     Application startup complete.
```

- vLLM version: v0.13.0
- vLLM main:
d68209402d

Signed-off-by: drslark <slarksblood@qq.com>

vllm_ascend/ascend_forward_context.py

@@ -72,13 +72,16 @@ def set_ascend_forward_context(
         # due to multiple warmups before actual capturing
         forward_context.capturing = False
 
+        # TODO: remove it when torch_npu.npu_mm_reduce_scatter_base supports tp_size >= 16.
+        mmrs_fusion = tp_world_size <= 8
+
         # set for sequence parallelism, 1000 is the batch size concurrency threshold
         # for enabling the flashcomm_v1 or sequence_parallelism feature.
         # Currently, it is an empirical value. In normal scenarios, if the concurrency
         # exceeds this threshold, the performance benefits can be maximized.
         # Conversely, if the concurrency is below the threshold,
         # the performance may degrade due to the switching of communication methods.
-        mmrs_fusion = True
+
         # main model and drafter model may have different architecture
         is_context_moe_model = is_drafter_moe_model(vllm_config) if is_draft_model else is_moe_model(vllm_config)
         if is_context_moe_model:
@@ -86,6 +89,7 @@ def set_ascend_forward_context(
             mmrs_fusion = False
         else:
             sp_enabled = enable_sp(vllm_config) and num_tokens is not None and num_tokens > 1000
+
         forward_context.mmrs_fusion = mmrs_fusion
         forward_context.num_tokens = num_tokens
         forward_context.sp_enabled = sp_enabled