Optimize the speed of the truncate_sequences function. (#28263)

* change truncate_sequences

* Update tokenization_utils_base.py

* change format

* fix when ids_to_move=0

* fix

* Update src/transformers/tokenization_utils_base.py
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

---------
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

src/transformers/tokenization_utils_base.py

@@ -3557,21 +3557,26 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
                 "truncation strategy. So the returned list will always be empty even if some "
                 "tokens have been removed."
             )
-            for _ in range(num_tokens_to_remove):
-                if pair_ids is None or len(ids) > len(pair_ids):
-                    if self.truncation_side == "right":
-                        ids = ids[:-1]
-                    elif self.truncation_side == "left":
-                        ids = ids[1:]
-                    else:
-                        raise ValueError("invalid truncation strategy:" + str(self.truncation_side))
-                else:
-                    if self.truncation_side == "right":
-                        pair_ids = pair_ids[:-1]
-                    elif self.truncation_side == "left":
-                        pair_ids = pair_ids[1:]
-                    else:
-                        raise ValueError("invalid truncation strategy:" + str(self.truncation_side))
+            len_pair_ids = len(pair_ids) if pair_ids is not None else 0
+            len_ids = len(ids)
+            first_remove = min(abs(len_pair_ids - len_ids), num_tokens_to_remove)
+            second_remove = num_tokens_to_remove - first_remove
+            if len_ids > len_pair_ids:
+                ids_to_move = first_remove + second_remove // 2
+                pair_ids_to_move = second_remove - second_remove // 2
+            else:
+                ids_to_move = second_remove // 2
+                pair_ids_to_move = first_remove + second_remove - (second_remove // 2)
+
+            if self.truncation_side == "right":
+                ids = ids[:-ids_to_move] if ids_to_move > 0 else ids
+                pair_ids = pair_ids[:-pair_ids_to_move] if pair_ids is not None and pair_ids_to_move > 0 else pair_ids
+            elif self.truncation_side == "left":
+                ids = ids[ids_to_move:]
+                pair_ids = pair_ids[pair_ids_to_move:] if pair_ids is not None else None
+            else:
+                raise ValueError("invalid truncation strategy:" + str(self.truncation_side))
+
         elif truncation_strategy == TruncationStrategy.ONLY_SECOND and pair_ids is not None:
             if len(pair_ids) > num_tokens_to_remove:
                 window_len = min(len(pair_ids), stride + num_tokens_to_remove)