fix: slicing returns DataProto not DataProtoItem (#718)

verl/protocol.py

@@ -197,7 +197,8 @@ class DataProto:
     def __getitem__(self, item):
         tensor_data = self.batch[item]
         non_tensor_data = {key: val[item] for key, val in self.non_tensor_batch.items()}
-        return DataProtoItem(batch=tensor_data, non_tensor_batch=non_tensor_data, meta_info=self.meta_info)
+        return_type = DataProto if isinstance(item, slice) else DataProtoItem
+        return return_type(batch=tensor_data, non_tensor_batch=non_tensor_data, meta_info=self.meta_info)
 
     def __getstate__(self):
         import io
@@ -603,7 +604,7 @@ import ray
 class DataProtoFuture:
     """
     DataProtoFuture aims to eliminate actual data fetching on driver. By doing so, the driver doesn't have to wait
-    for data so that asynchronous execution becomes possible. 
+    for data so that asynchronous execution becomes possible.
     DataProtoFuture contains a list of futures from another WorkerGroup of size world_size.
     - collect_fn is a Callable that reduces the list of futures to a DataProto
     - dispatch_fn is a Callable that partitions the DataProto into a list of DataProto of size world_size and then select