[UX] Add `-ep` shorthand for `--enable-expert-parallel` (#30890)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/engine/arg_utils.py

@@ -839,7 +839,9 @@ class EngineArgs:
             **parallel_kwargs["data_parallel_external_lb"],
         )
         parallel_group.add_argument(
-            "--enable-expert-parallel", **parallel_kwargs["enable_expert_parallel"]
+            "--enable-expert-parallel",
+            "-ep",
+            **parallel_kwargs["enable_expert_parallel"],
         )
         parallel_group.add_argument(
             "--all2all-backend", **parallel_kwargs["all2all_backend"]