[tokenizers] Several small improvements and bug fixes (#5287)

* avoid recursion in id checks for fast tokenizers

* better typings and fix #5232

* align slow and fast tokenizers behaviors for Roberta and GPT2

* style and quality

* fix tests - improve typings

src/transformers/tokenization_utils_base.py

@@ -607,7 +607,7 @@ class SpecialTokensMixin:
                         "special token {} has to be either str or AddedToken but got: {}".format(key, type(value))
                     )
 
-    def sanitize_special_tokens(self):
+    def sanitize_special_tokens(self) -> int:
         """ Make sure that all the special tokens attributes of the tokenizer (tokenizer.mask_token, tokenizer.cls_token, ...)
             are in the vocabulary. Add the missing ones to the vocabulary if needed.
 
@@ -616,7 +616,7 @@ class SpecialTokensMixin:
         """
         return self.add_tokens(self.all_special_tokens_extended, special_tokens=True)
 
-    def add_special_tokens(self, special_tokens_dict):
+    def add_special_tokens(self, special_tokens_dict: Dict[str, Union[str, AddedToken]]) -> int:
         """
         Add a dictionary of special tokens (eos, pad, cls...) to the encoder and link them
         to class attributes. If special tokens are NOT in the vocabulary, they are added
@@ -665,10 +665,14 @@ class SpecialTokensMixin:
             setattr(self, key, value)
 
             if key == "additional_special_tokens":
-                assert isinstance(value, (list, tuple)) and all(isinstance(t, str) for t in value)
+                assert isinstance(value, (list, tuple)) and all(
+                    isinstance(t, (str, AddedToken)) for t in value
+                ), f"Tokens {value} for key {key} should all be str or AddedToken instances"
                 added_tokens += self.add_tokens(value, special_tokens=True)
             else:
-                assert isinstance(value, str)
+                assert isinstance(
+                    value, (str, AddedToken)
+                ), f"Token {value} for key {key} should be a str or an AddedToken instance"
                 added_tokens += self.add_tokens([value], special_tokens=True)
 
         return added_tokens
@@ -809,26 +813,36 @@ class SpecialTokensMixin:
     @property
     def bos_token_id(self):
         """ Id of the beginning of sentence token in the vocabulary. Log an error if used while not having been set. """
+        if self._bos_token is None:
+            return None
         return self.convert_tokens_to_ids(self.bos_token)
 
     @property
     def eos_token_id(self):
         """ Id of the end of sentence token in the vocabulary. Log an error if used while not having been set. """
+        if self._eos_token is None:
+            return None
         return self.convert_tokens_to_ids(self.eos_token)
 
     @property
     def unk_token_id(self):
         """ Id of the unknown token in the vocabulary. Log an error if used while not having been set. """
+        if self._unk_token is None:
+            return None
         return self.convert_tokens_to_ids(self.unk_token)
 
     @property
     def sep_token_id(self):
         """ Id of the separation token in the vocabulary. E.g. separate context and query in an input sequence. Log an error if used while not having been set. """
+        if self._sep_token is None:
+            return None
         return self.convert_tokens_to_ids(self.sep_token)
 
     @property
     def pad_token_id(self):
         """ Id of the padding token in the vocabulary. Log an error if used while not having been set. """
+        if self._pad_token is None:
+            return None
         return self.convert_tokens_to_ids(self.pad_token)
 
     @property
@@ -839,11 +853,15 @@ class SpecialTokensMixin:
     @property
     def cls_token_id(self):
         """ Id of the classification token in the vocabulary. E.g. to extract a summary of an input sequence leveraging self-attention along the full depth of the model. Log an error if used while not having been set. """
+        if self._cls_token is None:
+            return None
         return self.convert_tokens_to_ids(self.cls_token)
 
     @property
     def mask_token_id(self):
         """ Id of the mask token in the vocabulary. E.g. when training a model with masked-language modeling. Log an error if used while not having been set. """
+        if self._mask_token is None:
+            return None
         return self.convert_tokens_to_ids(self.mask_token)
 
     @property