[model_cards]: 🇹🇷 Add new ELECTRA small and base models for Turkish
Hi,

this PR introduces model cards for new ELECTRA small and base models for Turkish 🇹🇷.

More information (checkpoint evaluation, downstream evaluation on Pos Tagging and NER, loss curves, TensorBoards) see [this repository](https://github.com/stefan-it/turkish-bert/tree/electra/electra).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4318?src=pr&el=h1) Report
> Merging [#4318](https://codecov.io/gh/huggingface/transformers/pull/4318?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/4bf5042240d33286460b83f3dbf9be77500faab3&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4318/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4318?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4318      +/-   ##
==========================================
+ Coverage   78.16%   78.17%   +0.01%     
==========================================
  Files         120      120              
  Lines       20005    20005              
==========================================
+ Hits        15636    15638       +2     
+ Misses       4369     4367       -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4318?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4318/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.93% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4318/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `41.09% <0.00%> (+0.28%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4318?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/4318?src=pr&el=footer). Last update [4bf5042...dc686bb](https://codecov.io/gh/huggingface/transformers/pull/4318?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
