Nlp cli fix endpoints
LGTM 👍 
With this PR users will be able to upload their own datasets and metrics.

As mentioned in #181, I had to use the new endpoints and revert the use of dataclasses (just in case we have changes in the API in the future).

We now distinguish commands for datasets and commands for metrics:
```bash
nlp-cli upload_dataset <path/to/dataset>
nlp-cli upload_metric <path/to/metric>
nlp-cli s3_datasets {rm, ls}
nlp-cli s3_metrics {rm, ls}
```

Does it sound good to you @julien-c @thomwolf ?