refactor: remove redundant if-condition and improve type correctness …

…for `convert_tokens_to_ids` (#34030)

* chore: remove redundant if-condition

* fix: import `Iterable`

src/transformers/tokenization_utils_fast.py

@@ -21,7 +21,7 @@
 import json
 import os
 from collections import defaultdict
-from typing import Any, Dict, List, Optional, Tuple, Union
+from typing import Any, Dict, Iterable, List, Optional, Tuple, Union
 
 import tokenizers.pre_tokenizers as pre_tokenizers_fast
 from tokenizers import Encoding as EncodingFast
@@ -326,20 +326,17 @@ def _convert_encoding(
 
         return encoding_dict, encodings
 
-    def convert_tokens_to_ids(self, tokens: Union[str, List[str]]) -> Union[int, List[int]]:
+    def convert_tokens_to_ids(self, tokens: Union[str, Iterable[str]]) -> Union[int, List[int]]:
         """
-        Converts a token string (or a sequence of tokens) in a single integer id (or a sequence of ids), using the
+        Converts a token string (or a sequence of tokens) in a single integer id (or a Iterable of ids), using the
         vocabulary.
 
         Args:
-            tokens (`str` or `List[str]`): One or several token(s) to convert to token id(s).
+            tokens (`str` or `Iterable[str]`): One or several token(s) to convert to token id(s).
 
         Returns:
             `int` or `List[int]`: The token id or list of token ids.
         """
-        if tokens is None:
-            return None
-
         if isinstance(tokens, str):
             return self._convert_token_to_id_with_added_voc(tokens)