Add checksums
### Checksums files

They are stored next to the dataset script in urls_checksums/checksums.txt.
They are used to check the integrity of the datasets downloaded files.
I kept the same format as tensorflow-datasets.
There is one checksums file for all configs.

### Load a dataset

When you do `load("squad")`, it will also download the checksums file and put it next to the script in nlp/datasets/hash/urls_checksums/checksums.txt.
It also verifies that the downloaded files checksums match the expected ones.

You can ignore checksum tests with `load("squad", ignore_checksums=True)` (under the hood it just adds `ignore_checksums=True` in the `DownloadConfig`)

### Test a dataset

There is a new command `nlp-cli test squad` that runs `download_and_prepare` to see if it runs ok, and that verifies that all the checksums match. Allowed arguments are `--name`, `--all_configs`, `--ignore_checksums` and `--register_checksums`.

### Register checksums

1. If the dataset has external dataset files

The command `nlp-cli test squad --register_checksums --all_configs` runs `download_and_prepare` on all configs to see if it runs ok, and it creates the checksums file.
You can also register one config at a time using `--name` instead ; the checksums file will be completed and not overwritten.

If the script is a local script, the checksum file is moved to urls_checksums/checksums.txt next to the local script, to enable the user to upload both the script and the checksums file afterwards with `nlp-cli upload squad`.

2. If the dataset files are all inside the directory of the dataset script

The user can directly do `nlp-cli upload squad --register_checksums`, as there is no need to download anything.
In this case however, all the dataset must be uploaded at once.

--

PS : it doesn't allow to register checksums for canonical datasets, the file has to be added manually on S3 for now (I guess ?)

Also I feel like we must be sure that this processes would not constrain too much any user from uploading its dataset.
Let me know what you think :)