support fused_moe_allgather_ep (#1335)

### What this PR does / why we need it?
support fused_moe_allgather_ep

### How was this patch tested?
It was tested by UT.

Signed-off-by: lyj-jjj <liuyingjun5@huawei.com>

vllm_ascend/envs.py

@@ -99,6 +99,11 @@ env_variables: Dict[str, Callable[[], Any]] = {
     # Whether to enable the trace recompiles from pytorch.
     "VLLM_ASCEND_TRACE_RECOMPILES":
     lambda: bool(int(os.getenv("VLLM_ASCEND_TRACE_RECOMPILES", '0'))),
+    # Whether to enable fused_experts_allgather_ep. MoeInitRoutingV3 and
+    # GroupedMatmulFinalizeRouting operators are combined to implement EP.
+    "VLLM_ENABLE_FUSED_EXPERTS_ALLGATHER_EP":
+    lambda: bool(int(os.getenv("VLLM_ENABLE_FUSED_EXPERTS_ALLGATHER_EP", '0'))
+                 ),
     "VLLM_ASCEND_ENABLE_DBO":
     lambda: bool(int(os.getenv("VLLM_ASCEND_ENABLE_DBO", '0'))),
     # Whether to enable the model execute time observe profile. Disable it when