[examples] consolidate summarization examples
Consolidating summarization examples of T5 & Bertabs models into one - [#3826](https://github.com/huggingface/transformers/issues/3826)
@sshleifer 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4837?src=pr&el=h1) Report
> Merging [#4837](https://codecov.io/gh/huggingface/transformers/pull/4837?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e33fdc93b4ecb571dd7a8002a74789ec8bfffc09&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4837/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4837?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4837   +/-   ##
=======================================
  Coverage   76.17%   76.18%           
=======================================
  Files         128      128           
  Lines       21497    21497           
=======================================
+ Hits        16375    16377    +2     
+ Misses       5122     5120    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4837?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4837/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.27% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4837/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.20% <0.00%> (+0.15%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4837?src=pr&el=footer). Last update [e33fdc9...ed4de25](https://codecov.io/gh/huggingface/transformers/pull/4837?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>That's really cool, thanks for working on this @aretius!<|||||>@sshleifer Thanks for approving the PR!
Also, I am really interested to contribute more. It would be really great for me to be offered more chance to contribute to the repo :)