Optimizes ByT5 tokenizer (#13119)

* Starting to optimize ByT5.

* Making ByT5Tokenizer faster.

* Even faster.

* Cleaning up.

src/transformers/models/byt5/tokenization_byt5.py

@@ -15,7 +15,6 @@
 """ Tokenization class for model ByT5."""
 
 
-import re
 import warnings
 from typing import Dict, List, Optional, Tuple
 
@@ -92,17 +91,21 @@ class ByT5Tokenizer(PreTrainedTokenizer):
             **kwargs,
         )
 
+        self._extra_ids = extra_ids
+
+        self._utf_vocab_size = 2 ** 8  # utf is 8 bits
+
         # define special tokens dict
         self.special_tokens_encoder: Dict[int, str] = {
             self.pad_token: 0,
             self.eos_token: 1,
             self.unk_token: 2,
         }
-        self.special_tokens_decoder: Dict[str, int] = {v: k for k, v in self.special_tokens_encoder.items()}
-
         self._num_special_tokens = len(self.special_tokens_encoder)
-        self._utf_vocab_size = 2 ** 8  # utf is 8 bits
-        self._extra_ids = extra_ids
+        n = len(additional_special_tokens)
+        for i, token in enumerate(additional_special_tokens):
+            self.special_tokens_encoder[token] = self.vocab_size + i - n - 1
+        self.special_tokens_decoder: Dict[str, int] = {v: k for k, v in self.special_tokens_encoder.items()}
 
     @property
     def vocab_size(self):
@@ -196,35 +199,16 @@ class ByT5Tokenizer(PreTrainedTokenizer):
 
     def _tokenize(self, text: str) -> List[str]:
         """Take as input a string and return a list of strings (tokens) for words/sub-words"""
-
-        def _sub_tokenize(sub_text):
-            character_list = list(sub_text)
-            utf_tokens_lists = [list(char.encode("utf-8")) for char in character_list]
-            sub_tokens = [chr(utf_token) for utf_tokens in utf_tokens_lists for utf_token in utf_tokens]
-            return sub_tokens
-
-        # split on special characters
-        pattern = f"({'|'.join(self.special_tokens_encoder.keys())})"
-        sub_texts = list(filter(None, re.split(pattern, text)))
-        tokens = []
-        for sub_text in sub_texts:
-            if sub_text in self.special_tokens_encoder.keys():
-                tokens += [sub_text]
-            else:
-                tokens += _sub_tokenize(sub_text)
-
+        tokens = list(text)
         return tokens
 
     def _convert_token_to_id(self, token):
         """Converts a token (str) in an id using the vocab."""
-        if token.startswith("<extra_id_"):
-            match = re.match(r"<extra_id_(\d+)>", token)
-            num = int(match.group(1))
-            token_id = self.vocab_size - num - 1
-        elif token in self.special_tokens_encoder:
+        if token in self.special_tokens_encoder:
             token_id = self.special_tokens_encoder[token]
-        elif len(token) > 1:
-            # token of length > 1 must be newly added tokens => set them to unk token
+        elif token in self.added_tokens_encoder:
+            token_id = self.added_tokens_encoder[token]
+        elif len(token) != 1:
             token_id = self.unk_token_id
         else:
             token_id = ord(token) + self._num_special_tokens
@@ -232,35 +216,23 @@ class ByT5Tokenizer(PreTrainedTokenizer):
 
     def _convert_id_to_token(self, index):
         """Converts an index (integer) in a token (str) using the vocab."""
-        if index < self._num_special_tokens:
+        if index in self.special_tokens_decoder:
             token = self.special_tokens_decoder[index]
-        elif index < self._utf_vocab_size + self._num_special_tokens:
-            token = chr(index - self._num_special_tokens)
         else:
-            token = f"<extra_id_{self.vocab_size - 1 - index}>"
+            token = chr(index - self._num_special_tokens)
         return token
 
     def convert_tokens_to_string(self, tokens):
         """Converts a sequence of tokens (string) in a single string."""
-
-        def _convert_sub_string(sub_chars):
-            byte_string = bytes([ord(char) for char in sub_chars])
-            return byte_string.decode("utf-8", errors="ignore")
-
         string = ""
-        sub_chars = []
         for token in tokens:
-            # if is special token
-            if len(token) > 1:
-                string += _convert_sub_string(sub_chars)
-                string += token
-                sub_chars = []
+            if token in self.special_tokens_decoder:
+                tok_string = self.special_tokens_decoder[token]
+            elif token in self.added_tokens_decoder:
+                tok_string = self.added_tokens_decoder[token]
             else:
-                sub_chars.append(token)
-
-        # add remaining chars
-        string += _convert_sub_string(sub_chars)
-
+                tok_string = token
+            string += tok_string
         return string
 
     # ByT5Tokenizer has no vocab file