fix dataloader performace decrease & unittest hanging. test=develop (#26739)

python/paddle/fluid/dataloader/dataloader_iter.py

@@ -608,6 +608,7 @@ class _DataLoaderIterMultiProcess(_DataLoaderIterBase):
             # in _send_idx but will not increase _rcvd_idx, so we check 
             # whether the worker is still alive here to skip the discarded
             # batch indices and increase _rcvd_idx
+            if self._dataset_kind == _DatasetKind.ITER:
                 while self._rcvd_idx < self._send_idx:
                     info = self._task_infos[self._rcvd_idx]
                     if len(info) == 2 or self._worker_status[info[0]]:
@@ -623,7 +624,8 @@ class _DataLoaderIterMultiProcess(_DataLoaderIterBase):
                         return None
                     continue
 
-            if len(self._task_infos[self._rcvd_idx]) == 2:
+            if self._rcvd_idx in self._task_infos and \
+                    len(self._task_infos[self._rcvd_idx]) == 2:
                 return self._task_infos.pop(self._rcvd_idx)[1]
 
             try: