Save Dataset as Sharded Parquet 
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))
```
### 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. 