[Bugfix] Add constraints for sequence parallelism (#4014)

### What this PR does / why we need it?
Add Add constraints for sequence parallelism for unsupported scenarios:
1. tp_size > 1
2. enable_expert_parallel must be True for MoE model

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?


- vLLM version: v0.11.0
- vLLM main:
83f478bb19

---------

Signed-off-by: realliujiaxu <realliujiaxu@163.com>

vllm_ascend/utils.py

@@ -659,6 +659,17 @@ def enable_sp(vllm_config=None) -> bool:
             # We retain the env VLLM_ASCEND_ENABLE_FLASHCOMM here for backward compatibility.
             or bool(int(os.getenv("VLLM_ASCEND_ENABLE_FLASHCOMM", '0'))))
 
+        if not _ENABLE_SP:
+            return _ENABLE_SP
+
+        assert vllm_config.parallel_config.tensor_parallel_size > 1, \
+            "Flash Comm v1 (Sequence Parallelism) is only supported when tp_size > 1."
+
+        assert (
+            not is_moe_model(vllm_config)
+            or vllm_config.parallel_config.enable_expert_parallel
+        ), "Flash Comm v1 (Sequence Parallelism) requires enable_expert_parallel=True for MoE models."
+
     return _ENABLE_SP