Imports for Roberta conversion appear to be outdated
## 🐛 Bug

<!-- Important information -->

I'm trying to convert a custom Roberta model (from fairseq checkpoints) to a Tensorflow model.

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

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

## To Reproduce

Steps to reproduce the behavior:


It is possible to load checkpoints saved from Roberta directly?  From the documentation, it looks like it should be possible, but when I run the following code

```python
from transformers import TFRobertaModel
model = TFRobertaModel.from_pretrained('checkpoint_best.pt', from_pt=True)
```

I get
```
UnicodeDecodeError                        Traceback (most recent call last)
<ipython-input-2-47fa7f7cf639> in <module>()
----> 1 model = TFRobertaModel.from_pretrained('checkpoint_best.pt', from_pt=True)

~/venvs/transformers-tf/lib/python3.6/site-packages/transformers/modeling_tf_utils.py in from_pretrained(cls, pretrained_model_name_or_path, *model_args, **kwargs)
    208                 cache_dir=cache_dir, return_unused_kwargs=True,
    209                 force_download=force_download,
--> 210                 **kwargs
    211             )
    212         else:

~/venvs/transformers-tf/lib/python3.6/site-packages/transformers/configuration_utils.py in from_pretrained(cls, pretrained_model_name_or_path, **kwargs)
    152
    153         # Load config
--> 154         config = cls.from_json_file(resolved_config_file)
    155
    156         if hasattr(config, 'pruned_heads'):

~/venvs/transformers-tf/lib/python3.6/site-packages/transformers/configuration_utils.py in from_json_file(cls, json_file)
    184         """Constructs a `BertConfig` from a json file of parameters."""
    185         with open(json_file, "r", encoding='utf-8') as reader:
--> 186             text = reader.read()
    187         return cls.from_dict(json.loads(text))
    188

/mnt/xfs1/sw/pkg/devel/python3/3.6.2/lib/python3.6/codecs.py in decode(self, input, final)
    319         # decode input (taking the buffer into account)
    320         data = self.buffer + input
--> 321         (result, consumed) = self._buffer_decode(data, self.errors, final)
    322         # keep undecoded input until the next call
    323         self.buffer = data[consumed:]

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0: invalid start byte
```
I'm guessing since the checkpoint is in a binary, I need to first convert this to a json format.
It looks like it should be done [here](https://github.com/huggingface/transformers/blob/master/transformers/convert_roberta_original_pytorch_checkpoint_to_pytorch.py) .  However, when I try to run that script, I get an error

```
(transformers-tf) [jmorton@pcn-7-01 checkpoints]$ python convert_roberta_original_pytorch_checkpoint_to_pytorch.py  --help
To use data.metrics please install scikit-learn. See https://scikit-learn.org/stable/index.html
Traceback (most recent call last):
  File "convert_roberta_original_pytorch_checkpoint_to_pytorch.py", line 26, in <module>
    from transformers import (BertConfig, BertEncoder,
ImportError: cannot import name 'BertEncoder'
```

From what I can tell, those imports are dated and will need to be fixed anyways.  


## Environment

* OS: Centos 7
* Python version: PY3.6
* PyTorch version: 1.2.0
* PyTorch Transformers version (or branch):2.0.0
* Using GPU ? Not yet
* Distributed of parallel setup ? Nope
* Any other relevant information:

Getting the same error here<|||||>Will investigate, thanks for reporting. (And Hi, @louismartin :)<|||||>I had a similar issue trying to load BioBERT and I figured out what was going on in my case, sharing just in case that's what's going on in your case. In my case I converted TF BioBERT checkpoint to pytorch model. In my case the (first) problem was that I didn't provide a path to the config file. 

My local scripts are adapted from the python code that runs Glue. I have a `--config_name` parameter that specifies the json file from which the configuration is loaded. If you don't provide that one, it tries to infer it by using the model_name_or_path - and that's what caused my problem. 

Once I specified the config file, I had another problem that had to do with the following: `model_name_or_path` is supposed to be the path where you store the other info for the models and the model files are expected to follow a certain naming convention (e.g., in my case, it was looking for a `pytorch_model.bin` file; there is a similar file name for TF). 

Hope this helps!

<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>This was superseded, and closed, by #1512