fix IterableDataset may block model when num_workers > 0. test=develop (#40541)

python/paddle/fluid/dataloader/dataloader_iter.py

@@ -564,6 +564,14 @@ class _DataLoaderIterMultiProcess(_DataLoaderIterBase):
                     self._rcvd_idx += 1
                     self._batches_outstanding -= 1
                 else:
+                    # NOTE: when _rcvd_idx catch up _send_idx, which means
+                    #       one of following:
+                    #       1. all 2 * num_workers batches have been loaded
+                    #          and stored in _blocking_queue
+                    #       2. all data drained
+                    #       we need to let _thread blocking at _data_queue
+                    #       get_data to inoccupy CPU, otherwise may occupy
+                    #       CPU time for model running
                     # NOTE: in persistent workers mode, do not check data
                     #       drained here, simply let it go to _data_queue
                     #       reading to get _ResumeIteration
@@ -573,7 +581,6 @@ class _DataLoaderIterMultiProcess(_DataLoaderIterBase):
                         #       may also be data in blocking queue
                         if self._batches_outstanding < len(self._places):
                             return None
-                        continue
 
             if self._rcvd_idx in self._task_infos and \
                     len(self._task_infos[self._rcvd_idx]) == 3: