[Feat] shared expert dp for deepseek_mtp (#3811)

### What this PR does / why we need it?
Support shared expert DP for deepseek_mtp feature. 
`shared_expert_dp` requires `SP==True`, with corresponding parameter
restrictions.
Previously, due to the coupling between `shared_expert_dp` and torchair,
and the removal of `deepseek_mtp` in vllm_ascend, shared expert dp of
deepseek_mtp was temporarily removed.
Currently, by performing the `reduce_scatter` on the input of
deepssek_mtp in `mtp_proposer.py`, we ensure that it matches the
dimensions of `input_embedding`, and then perform the `all_gather` on
the output of mtp.

### How was this patch tested?
baseline:
<img width="1184" height="692" alt="image"
src="https://github.com/user-attachments/assets/9680d53a-7b1d-481a-accc-b8f3dae2b9e3"
/>

enable shared_expert_dp and multistream_overlap_shared_expert:
<img width="1167" height="687" alt="image"
src="https://github.com/user-attachments/assets/2531d06b-dfda-4e24-8628-6f4b0f677ddc"
/>

TPOT: 48ms -> 45.4ms
Average TPS per rank: 117.6 -> 126.1


- vLLM version: v0.11.2
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.2

---------

Signed-off-by: chenmenglong <chenmenglong1@huawei.com>
Signed-off-by: zengran <zengran2@huawei.com>
Co-authored-by: zengran <zengran2@huawei.com>

vllm_ascend/utils.py

@@ -758,7 +758,7 @@ def dense_optim_enable() -> bool:
     return envs_ascend.VLLM_ASCEND_ENABLE_DENSE_OPTIMIZE
 
 
-def enable_sp(vllm_config=None) -> bool:
+def enable_sp(vllm_config=None, enable_shared_expert_dp: bool = False) -> bool:
     global _ENABLE_SP
     if _ENABLE_SP is None:
         if vllm_config is None:
@@ -772,6 +772,12 @@ 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 and enable_shared_expert_dp:
+            _ENABLE_SP = True
+            logger.info(
+                "shared_expert_dp requires enable_sp = True. has set enable_sp to True"
+            )
+
         if not _ENABLE_SP:
             return _ENABLE_SP