Read and write to HF Storage Buckets with load_dataset() and push_to_hub()

[lhoestq]

TODO:

• Dataset.push_to_hub
• DatasetDict.push_to_hub
• IterableDataset.push_to_hub
• IterableDatasetDict.push_to_hub
• load_dataset
• tests
• docs

e.g.

from datasets import load_dataset

ds.push_to_hub("buckets/username/my-bucket/dataset-name")

# and later
ds = load_dataset("buckets/username/my-bucket/dataset-name")

(also taking the opportunity to factorize the code a bit)

[HuggingFaceDocBuilderDev]

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