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

python/paddle/fluid/dataloader/dataloader_iter.py

@@ -608,22 +608,24 @@ 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
-            while self._rcvd_idx < self._send_idx:
-                info = self._task_infos[self._rcvd_idx]
-                if len(info) == 2 or self._worker_status[info[0]]:
-                    break
-                del self._task_infos[self._rcvd_idx]
-                self._rcvd_idx += 1
-                self._batches_outstanding -= 1
-            else:
-                # NOTE: _rcvd_idx and _send_idx only record batches among
-                #       workers, if batches among workers drained, there
-                #       may also be data in blocking queue
-                if self._batches_outstanding < len(self._places):
-                    return None
-                continue
-
-            if len(self._task_infos[self._rcvd_idx]) == 2:
+            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]]:
+                        break
+                    del self._task_infos[self._rcvd_idx]
+                    self._rcvd_idx += 1
+                    self._batches_outstanding -= 1
+                else:
+                    # NOTE: _rcvd_idx and _send_idx only record batches among
+                    #       workers, if batches among workers drained, there
+                    #       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]) == 2:
                 return self._task_infos.pop(self._rcvd_idx)[1]
 
             try: