[Tokenizer] batch_encode_plus method cannot encode List[Tuple[str]] with is_pretokenized=True
# 🐛 Bug

## Information

Model I am using: BERT

Language I am using the model on: English

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Load a tokenizer
2. Create a generic List[Tuple[List[int], List[int]]], for example [([2023, 2573], [2023, 2573, 2205])]
3. Encode the list using the method batch_encode_plus with is_tokenized=True to encode the pairs together
4. Error
```python
from transformers import BertTokenizer
import torch
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
text = [('This works','this works too')]
print(tokenizer.batch_encode_plus(text, add_special_tokens=False)['input_ids'])  # This works
input_ids = [([2023, 2573], [2023, 2573, 2205])]
tokenizer.batch_encode_plus(input_ids, is_pretokenized=True) # This raises error

/usr/local/lib/python3.6/dist-packages/transformers/tokenization_utils.py in get_input_ids(text)
   1715             else:
   1716                 raise ValueError(
-> 1717                     "Input is not valid. Should be a string, a list/tuple of strings or a list/tuple of integers."
   1718                 )
   1719 

ValueError: Input is not valid. Should be a string, a list/tuple of strings or a list/tuple of integers.
```

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## Expected behavior

batch_encode_plus would be able to encode List[Tuple[List[int], List[int]]], as described in the function description, hence, the example's input_ids would be:
`[[101, 2023, 2573, 102, 2023, 2573, 2205, 102]]`

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.11.0
- Platform: Linux-4.19.104+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.5.0+cu101 (True)
- Tensorflow version (GPU?): 2.2.0 (True)
- Using GPU in script?: probably not, but irrelevant
- Using distributed or parallel set-up in script?: probably not
Hi @vjeronymo2,  `encode_plus`  method expects list of `str` or `tokens`  if you are using `is_pretokenized=True`.
`tokens` here are not ints, they are wordpiece tokens, if you want to convert a string to tokens then you can use `.tokenize` method<|||||>> Hi @vjeronymo2, `encode_plus` method expects list of `str` or `tokens` if you are using `is_pretokenized=True`.
> `tokens` here are not ints, they are wordpiece tokens, if you want to convert a string to tokens then you can use `.tokenize` method

Thanks for clarifying that. I guess my biggest confusion was with the correct meaning of is_pretokenized flag.
Have a nice day, good sir.