[EncoderDecoder] Add xlm-roberta to encoder decoder
This PR adds `XLM-Roberta` to the EncoderDecoder framework by adding a new `XLMRobertaForCausalLM` to the models.

The XLM-Roberta EncoderDecoder can be used as follows:

```python 
from transformers import EncoderDecoderModel
import torch

model = EncoderDecoderModel.from_encoder_decoder_pretrained("xlm-roberta-base", "xlm-roberta-base")
input_ids = torch.tensor([10 * [0]])
outputs = model(input_ids, decoder_input_ids=input_ids, labels=input_ids, return_dict=True)
print("Loss", outputs.loss)
```

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6878?src=pr&el=h1) Report
> Merging [#6878](https://codecov.io/gh/huggingface/transformers/pull/6878?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3726754a6c646adcf9cb2135ab7f72dffe074473?el=desc) will **decrease** coverage by `3.21%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6878/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6878?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6878      +/-   ##
==========================================
- Coverage   80.05%   76.84%   -3.22%     
==========================================
  Files         157      157              
  Lines       28822    28825       +3     
==========================================
- Hits        23074    22150     -924     
- Misses       5748     6675     +927     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6878?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.28% <ø> (ø)` | |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `64.36% <ø> (-14.37%)` | :arrow_down: |
| [src/transformers/modeling\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG1fcm9iZXJ0YS5weQ==) | `100.00% <100.00%> (ø)` | |
| [...c/transformers/modeling\_tf\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `10.00% <0.00%> (-76.00%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsLnB5) | `19.85% <0.00%> (-68.29%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `16.25% <0.00%> (-63.52%)` | :arrow_down: |
| [src/transformers/configuration\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3RyYW5zZm9feGwucHk=) | `27.27% <0.00%> (-61.82%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `6.71% <0.00%> (-59.71%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `25.63% <0.00%> (-54.32%)` | :arrow_down: |
| [src/transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb25fdGYucHk=) | `33.33% <0.00%> (-24.33%)` | :arrow_down: |
| ... and [24 more](https://codecov.io/gh/huggingface/transformers/pull/6878/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6878?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6878?src=pr&el=footer). Last update [3726754...06cc500](https://codecov.io/gh/huggingface/transformers/pull/6878?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>cc @laibamehnaz <|||||>> cc @laibamehnaz

Thank you :)