[algo] fix: avoid FSDP _is_root assertion when collecting LoRA weights (#650)

Author: rank-Yu

verl/workers/sharding_manager/fsdp_vllm.py

@@ -121,6 +121,7 @@ def _collect_lora_weights(self) -> dict:
                 else:
                     lora_weights[key] = lora_weight.detach().cpu()
 
+            submodule._is_root = False
             if self.use_param_offload:
                 offload_fsdp_submodule(submodule)