MBartForConditionalGeneration
Author: patil-suraj
Pull request: yes

This PR adds MBartForConditionalGeneration. Regarding #6416 

@sshleifer 

Comments:
The failure is coming from `test_modeling_tf_electra.py`

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6441?src=pr&el=h1) Report
> Merging [#6441](https://codecov.io/gh/huggingface/transformers/pull/6441?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6e0b1dc8954b87c18f77a82000e81e02683b8eb1&el=desc) will **increase** coverage by `0.29%`.
> The diff coverage is `88.22%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6441/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6441?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6441      +/-   ##
==========================================
+ Coverage   79.77%   80.06%   +0.29%     
==========================================
  Files         148      156       +8     
  Lines       27214    28024     +810     
==========================================
+ Hits        21710    22438     +728     
- Misses       5504     5586      +82     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6441?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `100.00% <ø> (ø)` | |
| [src/transformers/data/test\_generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Rlc3RfZ2VuZXJhdGlvbl91dGlscy5weQ==) | `0.00% <0.00%> (ø)` | |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `90.00% <ø> (-0.91%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `87.35% <ø> (ø)` | |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `100.00% <ø> (+4.22%)` | :arrow_up: |
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `12.25% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/testing\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90ZXN0aW5nX3V0aWxzLnB5) | `51.92% <28.57%> (-20.81%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.84% <37.50%> (-0.18%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.38% <50.00%> (+1.79%)` | :arrow_up: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `90.90% <52.94%> (-5.68%)` | :arrow_down: |
| ... and [69 more](https://codecov.io/gh/huggingface/transformers/pull/6441/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6441?src=pr&el=footer). Last update [e92efcf...49f74a5](https://codecov.io/gh/huggingface/transformers/pull/6441?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Thanks @sgugger , I've applied the suggestions.