[Community notebooks] General notebooks
A proposal how we could link community notebooks.

I'm using the awesome notebook of @patil-suraj (`nlp` + `Trainer` + `transformers` :-)) as an example of how community models can be added. 

@patil-suraj - could you maybe review the PR and see whether it's ok for you?
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4441?src=pr&el=h1) Report
> Merging [#4441](https://codecov.io/gh/huggingface/transformers/pull/4441?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/590adb130be8e99eb638bb22136dda537b2da71d&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/4441/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4441?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4441   +/-   ##
=======================================
  Coverage   78.14%   78.15%           
=======================================
  Files         120      120           
  Lines       20087    20087           
=======================================
+ Hits        15697    15698    +1     
+ Misses       4390     4389    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4441?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.77% <0.00%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4441?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4441?src=pr&el=footer). Last update [590adb1...df8c3a1](https://codecov.io/gh/huggingface/transformers/pull/4441?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@patrickvonplaten This sounds good! Having a separate table for community models makes sense.  