Shard parquet in `download_and_prepare`
{'login': 'lhoestq', 'id': 42851186, 'node_id': 'MDQ6VXNlcjQyODUxMTg2', 'avatar_url': 'https://avatars.githubusercontent.com/u/42851186?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lhoestq', 'html_url': 'https://github.com/lhoestq', 'followers_url': 'https://api.github.com/users/lhoestq/followers', 'following_url': 'https://api.github.com/users/lhoestq/following{/other_user}', 'gists_url': 'https://api.github.com/users/lhoestq/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lhoestq/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lhoestq/subscriptions', 'organizations_url': 'https://api.github.com/users/lhoestq/orgs', 'repos_url': 'https://api.github.com/users/lhoestq/repos', 'events_url': 'https://api.github.com/users/lhoestq/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lhoestq/received_events', 'type': 'User', 'site_admin': False}
[]
[]
_The documentation is not available anymore as the PR was closed or merged._ This is ready for review cc @mariosasko :) please let me know what you think !
Following https://github.com/huggingface/datasets/pull/4724 (needs to be merged first)

It's good practice to shard parquet files to enable parallelism with spark/dask/etc.

I added the `max_shard_size` parameter to `download_and_prepare` (default to 500MB for parquet, and None for arrow).

```python
from datasets import *

output_dir = "./output_dir"  # also supports "s3://..."
builder = load_dataset_builder("squad")
builder.download_and_prepare(output_dir, file_format="parquet", max_shard_size="5MB")
```

### Implementation details

The examples are written to a parquet file until `ParquetWriter._num_bytes > max_shard_size`. When this happens, a new writer is instantiated to start writing the next shard. At the end, all the shards are renamed to include the total number of shards in their names: `{builder.name}-{split}-{shard_id:05d}-of-{num_shards:05d}.parquet`

I also added the `MAX_SHARD_SIZE` config variable (default to 500MB)

TODO:
- [x] docstrings
- [x] docs
- [x] tests

cc @severo 
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/4747/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/4747', 'html_url': 'https://github.com/huggingface/datasets/pull/4747', 'diff_url': 'https://github.com/huggingface/datasets/pull/4747.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/4747.patch', 'merged_at': '2022-09-15T13:41:26Z'}