ImportError: cannot import name 'RobertaLMHeadModel'
Hi all, I was just trying to run a text generation script for low-resource languages, and therefore experimented with XLM-R and initially with Roberta, using the documentation for RobertaForCausalLM:
here: https://huggingface.co/transformers/model_doc/roberta.html#robertaforcausallm

I am running into the import error shown in the title. See code snippet and error message below. I also experimented with different Tensorflow and transformers versions to no avail. I suspect that the model classes have changed (or the documentation may not be up to date with the current version). I also tried importing RobertaForCausalLM but it returned the same error.

## Environment info     
- `transformers` version: 3.1.0
- Platform: Linux-4.15.0-112-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- Tensorflow version: 2.3.1


### Who can help
@LysandreJik , @TevenLeScao
 
Model I am using (**Roberta**, **XLM-Roberta**):

The problem arises when using:
* [X] the official example scripts: (give details below)
* [ ] 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. Run:
```
from transformers import RobertaTokenizer, RobertaLMHeadModel, RobertaConfig
import torch
tokenizer = RobertaTokenizer.from_pretrained('roberta-base')
config = RobertaConfig.from_pretrained("roberta-base")
config.is_decoder = True
model = RobertaLMHeadModel.from_pretrained('roberta-base', config=config, return_dict=True)
inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")
outputs = model(**inputs)
prediction_logits = outputs.logits
```

Error: `ImportError: cannot import name 'RobertaLMHeadModel'`

If this script runs succesfully, I 'd like to re-run it for XMLRoberta (changing the imports and model names of course).
Many thanks!