Fast tokenizers can't `encode_plus` a list of ids; slow tokenizers can
With the slow tokenizers:
```
>>> import transformers
>>> t = transformers.AutoTokenizer.from_pretrained("bert-base-cased", use_fast=False)
>>> t.encode_plus([1000])
{'input_ids': [101, 1000, 102],
 'token_type_ids': [0, 0, 0],
 'attention_mask': [1, 1, 1]}
```

With the fast tokenizers:
```
>>> import transformers
>>> t = transformers.AutoTokenizer.from_pretrained("bert-base-cased", use_fast=True)
>>> t.encode_plus([1000])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/dirkg/anaconda3/envs/allennlp/lib/python3.7/site-packages/transformers/tokenization_utils.py", line 1889, in encode_plus
    **kwargs,
  File "/Users/dirkg/anaconda3/envs/allennlp/lib/python3.7/site-packages/transformers/tokenization_utils.py", line 1815, in batch_encode_plus
    tokens = self._tokenizer.encode(*batch_text_or_text_pairs[0])
  File "/Users/dirkg/anaconda3/envs/allennlp/lib/python3.7/site-packages/tokenizers/implementations/base_tokenizer.py", line 141, in encode
    return self._tokenizer.encode(sequence, pair)
TypeError
```

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