[wip] Pegasus: Hack to never generate unk
Author: sshleifer
Pull request: yes

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Fixes #{issue number}


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7014?src=pr&el=h1) Report
> Merging [#7014](https://codecov.io/gh/huggingface/transformers/pull/7014?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ce37be9d94da57897cce9c49b3421e6a8a927d4a?el=desc) will **increase** coverage by `2.40%`.
> The diff coverage is `20.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7014/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7014?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7014      +/-   ##
==========================================
+ Coverage   77.60%   80.01%   +2.40%     
==========================================
  Files         161      161              
  Lines       30120    30124       +4     
==========================================
+ Hits        23374    24103     +729     
+ Misses       6746     6021     -725     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7014?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_pegasus.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19wZWdhc3VzLnB5) | `69.23% <20.00%> (-30.77%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `25.13% <0.00%> (-73.83%)` | :arrow_down: |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `60.00% <0.00%> (-30.00%)` | :arrow_down: |
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `85.00% <0.00%> (-5.00%)` | :arrow_down: |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `93.90% <0.00%> (-0.51%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.97% <0.00%> (-0.33%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.76% <0.00%> (-0.28%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+2.50%)` | :arrow_up: |
| [src/transformers/modeling\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19seG1lcnQucHk=) | `90.76% <0.00%> (+20.74%)` | :arrow_up: |
| ... and [3 more](https://codecov.io/gh/huggingface/transformers/pull/7014/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7014?src=pr&el=footer). Last update [ce37be9...f841173](https://codecov.io/gh/huggingface/transformers/pull/7014?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).