Accumulate tokens into batches in `PreTrainedTokenizerBase.add_tokens()` (#17119)

* Accumulate tokens into batches in PreTrainedTokenizerBase.add_tokens()

For tokenizers with a small number of special tokens or special tokens
with consecutive token IDs, this reduces the time complexity of creating
the trie from quadratic to linear, see also #16936.

* Extend explanation of batching added tokens

src/transformers/tokenization_utils_base.py

@@ -1964,24 +1964,38 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
             # Sort added tokens by index
             added_tok_encoder_sorted = list(sorted(added_tok_encoder.items(), key=lambda x: x[1]))
 
+            # Accumulate added tokens into batches of special/non-special tokens, because calling add_tokens() for
+            # individual tokens would repeatedly rebuild a trie, which can be slow.
+            is_last_special = None
+            tokens = []
+
             for token, index in added_tok_encoder_sorted:
-                if has_tokenizer_file and index != len(tokenizer) and tokenizer.convert_tokens_to_ids(token) != index:
+                current_index = len(tokenizer) + len(tokens)
+                if has_tokenizer_file and index != current_index and tokenizer.convert_tokens_to_ids(token) != index:
                     # Tokenizer fast: added token needs to either be in the vocabulary with the proper index or the
                     # index is the current length of the tokenizer (not in vocabulary)
                     raise ValueError(
                         f"Wrong index found for {token}: should be {tokenizer.convert_tokens_to_ids(token)} but found "
                         f"{index}."
                     )
-                elif not has_tokenizer_file and index != len(tokenizer):
+                elif not has_tokenizer_file and index != current_index:
                     # Tokenizer slow: added token cannot already be in the vocabulary so its index needs to be the
                     # current length of the tokenizer.
                     raise ValueError(
                         f"Non-consecutive added token '{token}' found. "
-                        f"Should have index {len(tokenizer)} but has index {index} in saved vocabulary."
+                        f"Should have index {current_index} but has index {index} in saved vocabulary."
                     )
 
-                # Safe to call on a tokenizer fast even if token already there.
-                tokenizer.add_tokens(token, special_tokens=bool(token in special_tokens))
+                is_special = bool(token in special_tokens)
+                if is_last_special is None or is_last_special == is_special:
+                    tokens.append(token)
+                else:
+                    tokenizer.add_tokens(tokens, special_tokens=is_last_special)
+                    tokens = [token]
+                is_last_special = is_special
+
+            if tokens:
+                tokenizer.add_tokens(tokens, special_tokens=is_last_special)
 
         # Check all our special tokens are registered as "no split" token (we don't cut them) and are in the vocab
         added_tokens = tokenizer.sanitize_special_tokens()