Title: Save Dataset as Sharded Parquet 
State: open
Labels: description: New feature or request
name: enhancement

### Feature request

`to_parquet` currently saves the dataset as one massive, monolithic parquet file, rather than as several small parquet files. It should shard large datasets automatically.

### Motivation

This default behavior makes me very sad because a program I ran for 6 hours saved its results using `to_parquet`, putting the entire billion+ row dataset into a 171 GB *single shard parquet file* which pyarrow, apache spark, etc. all cannot work with without completely exhausting the memory of my system. I was previously able to work with larger-than-memory parquet files, but not this one. I *assume* the reason why this is happening is because it is a single shard. Making sharding the default behavior puts datasets in parity with other frameworks, such as spark, which automatically shard when a large dataset is saved as parquet. 

### Your contribution

I could change the logic here https://github.com/huggingface/datasets/blob/bf6f41e94d9b2f1c620cf937a2e85e5754a8b960/src/datasets/io/parquet.py#L109-L158
to use `pyarrow.dataset.write_dataset`, which seems to support sharding, or periodically open new files. We would only shard if the user passed in a path rather than file handle. 

Comments:
To anyone else who finds themselves in this predicament, it's possible to read the parquet file in the same way that datasets writes it, and then manually break it into pieces. Although, you need a couple of magic options (`thrift_*`) to deal with the huge metadata, otherwise pyarrow immediately crashes.
```python
import pyarrow.parquet as pq
import pyarrow as pa

r = pq.ParquetReader()

r.open("./outrageous-file.parquet",thrift_string_size_limit=2**31-1, thrift_container_size_limit=2**31-1)

from more_itertools import chunked
import tqdm

for i,chunk in tqdm.tqdm(enumerate(chunked(range(r.num_row_groups),10000))):
    w = pq.ParquetWriter(f"./chunks.parquet/chunk{i}.parquet",schema=r.schema_arrow)
    for idx in chunk:
        w.write_table(r.read_row_group(idx))
    w.close()
```

You can also use `.shard()` and call `to_parquet()` on each shard in the meantime:

```python
num_shards = 128
output_path_template = "output_dir/{index:05d}.parquet"
for index in range(num_shards):
    shard = ds.shard(index=index, num_shards=num_shards, contiguous=True)
    shard.to_parquet(output_path_template.format(index=index))
```

> You can also use `.shard()` and call `to_parquet()` on each shard in the meantime:
> 
> num_shards = 128
> output_path_template = "output_dir/{index:05d}.parquet"
> for index in range(num_shards):
>     shard = ds.shard(index=index, num_shards=num_shards, contiguous=True)
>     shard.to_parquet(output_path_template.format(index=index))

I understand that this approach requires loading the entire dataset into memory before sharding. What is the recommended best practice for handling extremely large datasets (for example, in the context of DCLM) that are too large to fit in memory?

`load_dataset()` doesn't load the dataset in memory, and sharding neither. Datasets in `datasets` are memory mapped from disk, which allows to load datasets bigger than RAM.

And if the dataset is even bigger than your disk, you could even use `streaming=True` which gives an IterableDataset.

Check out the [documentation](https://huggingface.co/docs/datasets) if you want to learn more about Dataset and IterableDatasetn, as well as `.push_to_hub()` which pushes datasets as sharded Parquet on Hugging Face automatically and supports parallelism with multiprocessing for large datasets.

is there any way to reshard an iterabledataset?

In general one shard = one file, so the simplest is to re export the dataset in multiple files, e.g. with `.push_to_hub(..., num_shards=...)`. However I noticed that IterableDataset.push_to_hub only accepts `num_shards<=dataset.num_shards` at the moment and should be improved. In the meantime you can still stream the dataset into a `pq.ParquetWriter` to write multiple files

I was also missing functionality to write a dataset to a sharded Parquet. I ended up writing a module that writes streaming data (which we can get from a dataset) to a sharded Parquet: https://github.com/apcamargo/parquet-stream-writer