weights not initialised in pre-trained Roberta
## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....): Robert

Language I am using the model on (English, Chinese....): English

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

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [X] my own task or dataset: I am importing pre-trained models

## To Reproduce

When running the following code:

```
import logging
logging.basicConfig(level=logging.INFO)
from transformers import RobertaForMaskedLM
pt_m = RobertaForMaskedLM.from_pretrained('roberta-base')
```
 
I am getting the following messages in the log:

```
INFO:transformers.modeling_utils:loading weights file https://s3.amazonaws.com/models.huggingface.co/bert/roberta-base-pytorch_model.bin from cache at /home/bartosz/.cache/torch/transformers/228756ed15b6d200d7cb45aaef08c087e2706f54cb912863d2efe07c89584eb7.49b88ba7ec2c26a7558dda98ca3884c3b80fa31cf43a1b1f23aef3ff81ba344e
INFO:transformers.modeling_utils:Weights of RobertaForMaskedLM not initialized from pretrained model: ['lm_head.decoder.weight']
INFO:transformers.modeling_utils:Weights from pretrained model not used in RobertaForMaskedLM: ['lm_head.weight']
```

In particular, I am concerned that the weight of the LM head were not initialised from the stored values.

<!-- If you have a code sample, error messages, stack traces, please provide it here as well. -->

## Expected behavior

The model should load and all weights should get initialized from the pre-trained model.

<!-- A clear and concise description of what you expected to happen. -->

## Environment

* OS: Linux
* Python version: 3.7
* PyTorch version: 1.2.1
* PyTorch Transformers version (or branch): 2.2.2
* Using GPU ? No
* Distributed of parallel setup ? No
* Any other relevant information:

## Additional context

If I comment out these two lines the messages disappear:

https://github.com/huggingface/transformers/blob/f061606277322a013ec2d96509d3077e865ae875/transformers/modeling_utils.py#L445-L446

<!-- Add any other context about the problem here. -->

Thanks for the bug report. Fixed on `master` in 9a399ead253e27792cbf0ef386cc39f9b7084f8f by reverting the output of #1778.