Add check to provide hicache-storage-backend when enabling kv caching on Decode Side in PD Disaggregation (#20732)

Signed-off-by: Mohd Muzzammil <me.muzzammil@samsung.com>

python/sglang/srt/server_args.py

@@ -3111,6 +3111,11 @@ class ServerArgs:
                 raise ValueError(
                     "The argument disaggregation-decode-enable-offload-kvcache is only supported for decode side."
                 )
+            if self.hicache_storage_backend is None:
+                raise ValueError(
+                    "The argument disaggregation-decode-enable-offload-kvcache is only supported when hicache-storage-backend is provided."
+                )
+
         if not (0 < self.swa_full_tokens_ratio <= 1.0):
             raise ValueError("--swa-full-tokens-ratio should be in range (0, 1.0].")