

TITLE
Cannot load saved tokenizer using AutoTokenizer

COMMENTS
5

REACTIONS
+1: 0
-1: 0
laugh: 0
hooray: 0
heart: 0
rocket: 0
eyes: 0

BODY
## 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: 3.4.0
- Platform: Win10 x64 (1607 Build 14393.3866)
- Python version: 3.6.10
- PyTorch version (GPU?): 1.5.1
- Tensorflow version (GPU?): None
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
<!-- Your issue will be replied to more quickly if you can figure out the right person to tag with @
 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.
 
 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Speed and Memory Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 Translation: @sshleifer
 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @TevenLeScao
 blenderbot: @mariamabarham
 Bart: @sshleifer
 Marian: @sshleifer
 T5: @patrickvonplaten
 Longformer/Reformer: @patrickvonplaten
 TransfoXL/XLNet: @TevenLeScao 
 examples/seq2seq: @sshleifer
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
 documentation: @sgugger
 -->
@mfuntowicz
## Information

It appears that you can save a tokenizer to disk in a model agnostic way, but you cannot load it back in a model agnostic way. Is this a bug or by design?

## To reproduce

Steps to reproduce the behavior:

```
tokenizer = AutoTokenizer.from_pretrained('roberta-base')
tokenizer.save_pretrained('TEST/tokenizer')

tokenizer = AutoTokenizer.from_pretrained('TEST/tokenizer')
# ERROR
```

The error you get is because the config argument is None, which means AutoTokenizer calls AutoConfig.from_pretrained, which utilises file_utils.CONFIG_NAME, however tokenizer.save_pretrained uses tokenization_utils_base.TOKENIZER_CONFIG_FILE instead, so they're not compatible with one another.


## Expected behavior

I would assume that calling AutoTokenizer.from_pretrained would be able to load and instantiate the correct model tokenizer without the user having to directly import the model tokenizer class first (e.g. RobertaTokenizer.from_pretrained). This would help a lot in moving to a model agnostic way of handling tokenizers, which I feel is the goal of the AutoTokenizer class. The fact that it can't load a tokenizer from disk seems to be a bug, unless there is a different way of doing this?
