dataset(wlasl): initial loading script
Followup: 
From the info in https://github.com/huggingface/datasets/pull/722, I probably should load the videos as array of frames directly into the database. 
This will make the dataset generation time very long, but will make working with the dataset much easier. When I run:
```
python datasets-cli dummy_data datasets/wlasl
```

I get:
```
Checking datasets/wlasl/wlasl.py for additional imports. 
Found main folder for dataset datasets/wlasl/wlasl.py at /home/nlp/amit/.cache/huggingface/modules/datasets_modules/datasets/wlasl 
Found specific version folder for dataset datasets/wlasl/wlasl.py at /home/nlp/amit/.cache/huggingface/modules/datasets_modules/datasets/wlasl/f0cad785350d770804f20c471b0cff2d7e3c7b210b5c3a228c393abb95a04786  
Found script file from datasets/wlasl/wlasl.py to /home/nlp/amit/.cache/huggingface/modules/datasets_modules/datasets/wlasl/f0cad785350d770804f20c471b0cff2d7e3c7b210b5c3a228c393abb95a04786/wlasl.py 
Found dataset infos file from datasets/wlasl/dataset_infos.json to /home/nlp/amit/.cache/huggingface/modules/datasets_modules/datasets/wlasl/f0cad785350d770804f20c471b0cff2d7e3c7b210b5c3a228c393abb95a04786/dataset_infos.json 
Found metadata file for dataset datasets/wlasl/wlasl.py at /home/nlp/amit/.cache/huggingface/modules/datasets_modules/datasets/wlasl/f0cad785350d770804f20c471b0cff2d7e3c7b210b5c3a228c393abb95a04786/wlasl.json 
Using custom data configuration default 
Loading Dataset Infos from /home/nlp/amit/.cache/huggingface/modules/datasets_modules/datasets/wlasl/f0cad785350d770804f20c471b0cff2d7e3c7b210b5c3a228c393abb95a04786
Creating dummy folder structure for datasets/wlasl/dummy/0.3.0...   
Dataset datasets with config None seems to already open files in the method `_split_generators(...)`. You might consider to instead only open files in the method `_generate_examples(...)` instead. If this is not possible the dummy data has to be created with less guidance. Make sure you create the file dummy_data. 
Traceback (most recent call last):      
File "datasets-cli", line 36, in         
service.run()                                                                                                                                                                                                                                                                                                                                                                                                            File "/home/nlp/amit/anaconda2/envs/meta-scholar/lib/python3.7/site-packages/datasets-1.1.2-py3.7.egg/datasets/commands/dummy_data.py", line 73, in run 
for split in generator_splits: 
UnboundLocalError: local variable 'generator_splits' referenced before assignment
```
takes like 9-10 hours to download all of the videos for the dataset, but it does finish :) 