[Weight Cache] Support static DP/EP layouts (#33684)

Co-authored-by: siyu <liusy58@linux.alibaba.com>

python/sglang/srt/server_args.py

@@ -8179,6 +8179,11 @@ class ServerArgs:
                 "(--weight-cache-mode off) for this configuration."
             )
 
+        if self.weight_cache_mode != "off" and self.enable_eplb:
+            raise ValueError(
+                "--weight-cache-mode is not supported together with --enable-eplb."
+            )
+
     def _is_mistral_native_format(self) -> bool:
         """True iff the checkpoint requires load_format=mistral.