[misc] fix: load and offload in compute log prob (#208)

- As titled
- Relevant: https://github.com/volcengine/verl/issues/181

verl/workers/fsdp_workers.py

@@ -470,6 +470,10 @@ class ActorRolloutRefWorker(Worker):
     @register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO)
     def compute_log_prob(self, data: DataProto):
         assert self._is_actor
+        if self._is_offload_param:
+            load_fsdp_param_and_grad(module=self.actor_module_fsdp,
+                                     device_id=torch.cuda.current_device(),
+                                     load_grad=self._is_offload_grad)
         data = data.to('cuda')
         # we should always recompute old_log_probs when it is HybridEngine
         data.meta_info['micro_batch_size'] = self.config.rollout.log_prob_micro_batch_size_per_gpu
@@ -491,7 +495,13 @@ class ActorRolloutRefWorker(Worker):
         if self.world_size > 1:
             self.actor.actor_module._handle.reshard(True)
 
+        if self._is_offload_param:
+            # NOTE(sgm): the grad is already in CPU, only offload param here
+            offload_fsdp_param_and_grad(module=self.actor_module_fsdp, offload_grad=self._is_offload_grad)
+
+        # clear kv cache
         torch.cuda.empty_cache()
+        log_gpu_memory_usage('After compute_log_prob', logger=logger)
         return output
 
     @register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO)