Tokenizers bug: version 2.10 doesn't honor `max_len` when instantiating a pretrained model
# 🐛 Bug

## Information

Hello! I've just upgraded from Transformers 2.8 to Transformers 2.10, and noticed that parameter `max_len` is not properly honored when instantiating a pretrained model. For example, in Transformer 2.8.0, I was able to limit the length of a tokenized sequence as follows:

```python
import transformers

>>> tok = transformers.RobertaTokenizer.from_pretrained('roberta-base', max_len=16)
>>> tok.encode('This is a sentence', pad_to_max_length=True)

[0, 152, 16, 10, 3645, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

>>>print(tok.max_len)

16
```

However, on version 2.10, `max_len` is ignored when loading a pretrained tokenizer:

```python
import transformers

>>> tok = transformers.RobertaTokenizer.from_pretrained('roberta-base', max_len=16)
>>> tok.encode('This is a sentence', pad_to_max_length=True)

[0, 152, 16, 10, 3645, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, ...] # 512 tokens

>>>print(tok.max_len)

512
```

This bug can be temporary solved by using `model_max_length` instead of `max_len`, but it broke the all my scripts that relied on that attribute. It seems that this issue was introduced in a recent change in [`tokenization_utils.py`](https://github.com/huggingface/transformers/blob/master/src/transformers/tokenization_utils.py) (line 825):

```python
# For backward compatibility we fallback to set model_max_length from max_len if provided
model_max_length = model_max_length if model_max_length is not None else kwargs.pop("max_len", None)
```

This compatibility is not guaranteed if the pretrained model contains `model_max_length` among its parameters, but `max_len` is specified in `from_pretrained`. 

Model I am using (Bert, XLNet ...): As far as I can tell, this affects all pretrained models. Observed on BERT, RoBERTa, and DistilBERT. 

Language I am using the model on (English, Chinese ...): As far as I can tell, this affects all pretrained models. Observed on English. 

The problem arises when using:
* [ ] the official example scripts
* [x] my own modified scripts: See above. 

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task:
* [x] my own task or dataset: It's a classification task.

## To reproduce

See above.

## Expected behavior

See above.

## 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: 2.10.0
- Platform: Linux-4.15.0-1060-aws-x86_64-with-debian-buster-sid
- Python version: 3.6.5
- PyTorch version (GPU?): 1.4.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: yes; 4 x Tesla V100
- Using distributed or parallel set-up in script?: parallel, but not relevant

Additional info: I ran git-blame, and determined that the change was introduced in PR [#3706](https://github.com/huggingface/transformers/pull/3706). <|||||>Hi @soldni, thanks for reporting the issue.

The behavior you mention can now be achieved through: 

```python
>>> tok.encode('This is a sentence', pad_to_max_length=True, max_length=16)
[0, 152, 16, 10, 3645, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
```

Also, please note `RobertaTokenizer` now has it "fast" counterpart, `RobertaTokenizerFast` which is implemented in Rust and can greatly improve the performances of the tokenizer. API stays the same between both implementations.

If I'm not mistaken, the name was changed because it was misleading in the context of generation (i.e. `generate(...)`).

Morgan<|||||>Hi @mfuntowicz!

Thank you for the quick response! The issue remains that the tokenizer fails to initialize properly without raising an error. I guess I don't understand why `max_len` is still supported in some situations, but not others. I would have been fine with an error being raised, but hunting for this issue took quite a bit of time. 

-Luca  <|||||>You're right about the conflict if both are provided. 

I've opened a PR to at least write a warning the `max_len` parameter is being deprecated and `model_max_length` is now preferred.<|||||>Awesome! In the meantime, I've updated my code as you recommended. 

Thanks again for the super quick response on this. 

-Luca <|||||>@soldni I've fixed the issue when both are provided in the same PR, it will be included in the next patch release.

Thanks for reporting! I'm closing, feel free to reopen if needed 👍 
Morgan