[BART] Remove unused kwargs
Author: sshleifer
Pull request: yes

This doesn't change anything, 
- k_dim and v_dim kwargs are there for other models in fairseq but we don't need them.
- attention weights are returned by the AttentionModule (and ignored later) no matter what

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3279?src=pr&el=h1) Report
> Merging [#3279](https://codecov.io/gh/huggingface/transformers/pull/3279?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3814e167d99c4b2e135b250d73deaa3f63ebef0c&el=desc) will **decrease** coverage by `0.07%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3279/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3279?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3279      +/-   ##
==========================================
- Coverage   78.02%   77.94%   -0.08%     
==========================================
  Files          98       98              
  Lines       16670    16666       -4     
==========================================
- Hits        13007    12991      -16     
- Misses       3663     3675      +12     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3279?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3279/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `96.26% <100.00%> (-0.04%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3279/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.22% <0.00%> (-1.97%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3279/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `93.72% <0.00%> (-0.14%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3279?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/3279?src=pr&el=footer). Last update [3814e16...1b8aa30](https://codecov.io/gh/huggingface/transformers/pull/3279?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


I found one more `forward` in bertabs, these others are not obviously wrong.

`git grep "\.forward("`

```
examples/ner/run_pl_ner.py:        outputs = self.forward(**inputs)
examples/ner/run_pl_ner.py:        outputs = self.forward(**inputs)
```
on a lightning module so OK

```
examples/summarization/bertabs/modeling_bertabs.py:        See :obj:`onmt.modules.RNNDecoderBase.forward()` 
``` 
In documentation so OK

```
src/transformers/modeling_bart.py:        return super().forward(positions)
src/transformers/modeling_roberta.py:        return super().forward(
```
tried to change and got "super() is not callable".

Merging!