Issue: muchocine dataset cannot be dowloaded

```python
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
/usr/local/lib/python3.6/dist-packages/datasets/load.py in prepare_module(path, script_version, download_config, download_mode, dataset, force_local_path, **download_kwargs)
    267         try:
--> 268             local_path = cached_path(file_path, download_config=download_config)
    269         except FileNotFoundError:

7 frames
FileNotFoundError: Couldn't find file at https://raw.githubusercontent.com/huggingface/datasets/1.0.2/datasets/muchocine/muchocine.py

During handling of the above exception, another exception occurred:

FileNotFoundError                         Traceback (most recent call last)
FileNotFoundError: Couldn't find file at https://s3.amazonaws.com/datasets.huggingface.co/datasets/datasets/muchocine/muchocine.py

During handling of the above exception, another exception occurred:

FileNotFoundError                         Traceback (most recent call last)
/usr/local/lib/python3.6/dist-packages/datasets/load.py in prepare_module(path, script_version, download_config, download_mode, dataset, force_local_path, **download_kwargs)
    281                 raise FileNotFoundError(
    282                     "Couldn't find file locally at {}, or remotely at {} or {}".format(
--> 283                         combined_path, github_file_path, file_path
    284                     )
    285                 )

FileNotFoundError: Couldn't find file locally at muchocine/muchocine.py, or remotely at https://raw.githubusercontent.com/huggingface/datasets/1.0.2/datasets/muchocine/muchocine.py or https://s3.amazonaws.com/datasets.huggingface.co/datasets/datasets/muchocine/muchocine.py
```

Comments:
I have encountered the same error with `v1.0.1` and `v1.0.2` on both Windows and Linux environments. However, cloning the repo and using the path to the dataset's root directory worked for me. Even after having the dataset cached - passing the path is the only way (for now) to load the dataset.

```python
from datasets import load_dataset

dataset = load_dataset("squad")                      # Works
dataset = load_dataset("code_search_net", "python")  # Error
dataset = load_dataset("covid_qa_deepset")           # Error

path = "/huggingface/datasets/datasets/{}/"
dataset = load_dataset(path.format("code_search_net"), "python")  # Works
dataset = load_dataset(path.format("covid_qa_deepset"))           # Works
```



Hi @mrm8488 and @amoux!
 The datasets you are trying to load have been added to the library during the community sprint for v2 last month. They will be available with the v2 release!
For now, there are still a couple of solutions to load the datasets:
1. As suggested by @amoux, you can clone the git repo and pass the local path to the script
2. You can also install the latest (master) version of `datasets` using pip: `pip install git+https://github.com/huggingface/datasets.git@master`

If you don't want to clone entire `datasets` repo, just download the `muchocine` directory and pass the local path to the directory. Cheers!

Muchocine was added recently, that's why it wasn't available yet.

To load it you can just update `datasets`
```
pip install --upgrade datasets
```

and then you can load `muchocine` with

```python
from datasets import load_dataset

dataset = load_dataset("muchocine", split="train")
```

Thanks @lhoestq