[bugfix] fix megatron cp (real_position_ids) (#5683)

Author: Jintao-Huang

swift/llm/template/base.py

@@ -1682,6 +1682,8 @@ def _data_collator(self, batch: List[Dict[str, Any]], *, padding_to: Optional[in
                 cp_size = self.sequence_parallel_size
                 if cp_size > 1:
                     for key in ['position_ids', 'real_position_ids']:
+                        if key not in res:
+                            continue
                         padding_len = padding_to - seq_lens[0]
                         position_ids = res[key][0]
                         extended_position_ids = torch.arange(cp_size * 2).repeat(padding_len // (cp_size * 2))