`TypeError: TextInputSequence must be str` from Fast Tokenizer
### Bug

On version 4.5.1, trying to use fast tokenizer for Roberta, and got the above error. Weirdly, saw `transformers/models/gpt2/tokenization_gpt2_fast.py` in the traceback even no `gpt2` models are involved. 

**Script to reproduce**

```
from transformers import AutoTokenizer
from transformers.data.processors.utils import InputExample

if __name__ == "__main__":
    tokenizer = AutoTokenizer.from_pretrained(
        "roberta-base", use_fast=True
    )

    MAX_LENGTH = 256
    LABEL_LIST = [0, 1]
    OUTPUT_MODE = "classificaiton"

    inputs = ["Ututu goes public.", "This moon is huge."]
    examples = [InputExample(guid=str(index), text_a=text, label=None) for index, text in enumerate(inputs)]

    # this throws TypeError: TextInputSequence must be str
    batch_encoding = tokenizer(
        [(example.text_a, example.text_b) for example in examples],
        max_length=MAX_LENGTH,
        padding='max_length',
        truncation=True
    )
    print(batch_encoding)
```

**Traceback**

```
Traceback (most recent call last):
  File "tests/integration/tmp_test.py", line 22, in <module>
    truncation=True
  File "/usr/local/lib/python3.7/site-packages/transformers/tokenization_utils_base.py", line 2271, in __call__
    **kwargs,
  File "/usr/local/lib/python3.7/site-packages/transformers/tokenization_utils_base.py", line 2456, in batch_encode_plus
    **kwargs,
  File "/usr/local/lib/python3.7/site-packages/transformers/models/gpt2/tokenization_gpt2_fast.py", line 163, in _batch_encode_plus
    return super()._batch_encode_plus(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/transformers/tokenization_utils_fast.py", line 388, in _batch_encode_plus
    is_pretokenized=is_split_into_words,
TypeError: TextInputSequence must be str
```

Saw a similar issue raised for QnA pipeline, but I'm not doing QnA here. Thoughts?
You're sending this to your tokenizer: `[(example.text_a, example.text_b) for example in examples]`

But this is: `[('Ututu goes public.', None), ('This moon is huge.', None)]`.

A tokenizer cannot handle `None`, only text.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.