[Fix]Fix attention mask in default_collate_fn (#567)

fix attn mask

xtuner/dataset/collate_fns/default_collate_fn.py

@@ -39,6 +39,7 @@ def default_collate_fn(instances: Sequence[Dict],
         if has_image:
             pixel_values.append(example['pixel_values'])
 
+    ori_length = [len(ids) for ids in input_ids]
     if len(instances) > 1:
         input_ids = pad_sequence(
             input_ids, batch_first=True, padding_value=pad_index)
@@ -53,7 +54,12 @@ def default_collate_fn(instances: Sequence[Dict],
         attention_mask = None
         position_ids = torch.stack(position_ids, dim=0)
     else:
-        attention_mask = input_ids.ne(pad_index)
+        # Some tokenizers have the same eos token and pad token, so input_ids
+        # cannot be masked directly based on the pad token id.
+        attention_mask = torch.zeros_like(input_ids).bool()
+        for i in ori_length:
+            attention_mask[:i] = True
+
         bs, seq_len = input_ids.shape
         position_ids = torch.arange(seq_len).unsqueeze(0).long().repeat(bs, 1)