Add BlenderBot small tokenizer to the init
This class was forgotten, adding it to the init and the documentation.
Actually this tokenizer seems a bit broken:
```py
tokenizer = AutoTokenizer.from_pretrained("facebook/blenderbot_small-90M")
```
```
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/home/lysandre/Workspaces/Python/transformers/src/transformers/models/auto/tokenization_auto.py", line 469, in from_pretrained
    return tokenizer_class_fast.from_pretrained(pretrained_model_name_or_path, *inputs, **kwargs)
  File "/home/lysandre/Workspaces/Python/transformers/src/transformers/tokenization_utils_base.py", line 1741, in from_pretrained
    return cls._from_pretrained(
  File "/home/lysandre/Workspaces/Python/transformers/src/transformers/tokenization_utils_base.py", line 1858, in _from_pretrained
    tokenizer = cls(*init_inputs, **init_kwargs)
  File "/home/lysandre/Workspaces/Python/transformers/src/transformers/models/blenderbot_small/tokenization_blenderbot_small_fast.py", line 76, in __init__
    ByteLevelBPETokenizer(
  File "/home/lysandre/transformers/.env/lib/python3.8/site-packages/tokenizers/implementations/byte_level_bpe.py", line 36, in __init__
    BPE(
Exception: Error while initializing BPE: Token `_</w>` out of vocabular
```<|||||>Which only appears after fixing with 6d90d5a3213b812f3d54d6142751a275b59343b1

cc @patil-suraj if I recall correctly you implemented this tokenizer, do you remember what might have gone wrong?