Add LER


Comments:
Thanks for the comments. I addressed them and pushed again.
When I run "make quality" I get the following error but I don't know how to resolve it or what the problem ist respectively:
would reformat /Users/joelniklaus/NextCloud/PhDJoelNiklaus/Code/datasets/datasets/ler/ler.py
Oh no! 💥 💔 💥
1 file would be reformatted, 257 files would be left unchanged.
make: *** [quality] Error 1


Awesome thanks :)
To automatically format the python files you can run `make style`

I did that now. But still getting the following error:
black --check --line-length 119 --target-version py36 tests src benchmarks datasets metrics
All done! ✨ 🍰 ✨
258 files would be left unchanged.
isort --check-only tests src benchmarks datasets metrics
flake8 tests src benchmarks datasets metrics
datasets/ler/ler.py:46:96: W291 trailing whitespace
datasets/ler/ler.py:47:68: W291 trailing whitespace
datasets/ler/ler.py:48:102: W291 trailing whitespace
datasets/ler/ler.py:49:112: W291 trailing whitespace
datasets/ler/ler.py:50:92: W291 trailing whitespace
datasets/ler/ler.py:51:116: W291 trailing whitespace
datasets/ler/ler.py:52:84: W291 trailing whitespace
make: *** [quality] Error 1

However: When I look at the file I don't see any trailing whitespace

maybe a bug with flake8 ? could you try to update it ? which version do you have ?

This is my flake8 version: 3.7.9 (mccabe: 0.6.1, pycodestyle: 2.5.0, pyflakes: 2.1.1) CPython 3.8.5 on Darwin


Now I updated to: 3.8.4 (mccabe: 0.6.1, pycodestyle: 2.6.0, pyflakes: 2.2.0) CPython 3.8.5 on Darwin

And now I even get additional errors:
black --check --line-length 119 --target-version py36 tests src benchmarks datasets metrics
All done! ✨ 🍰 ✨
258 files would be left unchanged.
isort --check-only tests src benchmarks datasets metrics
flake8 tests src benchmarks datasets metrics
datasets/polyglot_ner/polyglot_ner.py:123:64: F541 f-string is missing placeholders
datasets/ler/ler.py:46:96: W291 trailing whitespace
datasets/ler/ler.py:47:68: W291 trailing whitespace
datasets/ler/ler.py:48:102: W291 trailing whitespace
datasets/ler/ler.py:49:112: W291 trailing whitespace
datasets/ler/ler.py:50:92: W291 trailing whitespace
datasets/ler/ler.py:51:116: W291 trailing whitespace
datasets/ler/ler.py:52:84: W291 trailing whitespace
datasets/math_dataset/math_dataset.py:233:25: E741 ambiguous variable name 'l'
metrics/coval/coval.py:236:31: F541 f-string is missing placeholders
make: *** [quality] Error 1

I do this on macOS Catalina 10.15.7 in case this matters

Code quality test now passes, thanks :) 

To fix the other tests failing I think you can just rebase from master.
Also make sure that the dummy data test passes with
```python
RUN_SLOW=1 pytest tests/test_dataset_common.py::LocalDatasetTest::test_load_dataset_all_configs_ler
```

I will close this PR because abishek did the same better (https://github.com/huggingface/datasets/pull/944)

Sorry you had to close your PR ! It looks like this week's sprint doesn't always make it easy to see what's being added/what's already added. 
Thank you for contributing to the library. You did a great job on adding LER so feel free to add other ones that you would like to see in the library, it will be a pleasure to review