Shard generator

Hi everyone! I was using Hugging Face datasets to process some very large datasets and found that it would be quite handy to have a feature that will allow to "split" these large datasets into chunks with equal size. Even better - be able to run through these chunks one by one in simple and convenient way. So I decided to add the method called shard_generator() to the main Dataset class. It works similar to shard method but it returns a generator of datasets with equal size (defined by shard_size attribute).
Example:
```python
>>> from datasets import load_dataset
>>> ds = load_dataset("rotten_tomatoes", split="validation")
>>> ds
Dataset({
      features: ['text', 'label'],
      num_rows: 1066
})
>>> next(ds.shard_generator(300))
Dataset({
      features: ['text', 'label'],
      num_rows: 300
})
```
I hope it can be helpful to someone. Thanks!

Comments:
Hi, thanks!

> I was using Hugging Face datasets to process some very large datasets and found that it would be quite handy to have a feature that will allow to "split" these large datasets into chunks with equal size

`map`, the method we use for processing in `datasets`, already does that if `batched=True`. And you can control the batch size with `batch_size`.

> Even better - be able to run through these chunks one by one in simple and convenient way

It's not hard to do this "manually" with the existing API:
```python
batch_size = <BATCH_SIZE>
for i in range(len(dset) // batch_size)
    shard = dset[i * batch_size:(i+1) * batch_size] # a dict of lists
    shard = Dataset.from_dict(shard)
```
(should be of similar performance to your implementation)

Still, I think an API like that could be useful if implemented efficiently (see [this](https://discuss.huggingface.co/t/why-is-it-so-slow-to-access-data-through-iteration-with-hugginface-dataset/20385) discussion to understand what's the issue with `select`/`__getitem__` on which your implementation relies on), which can be done with `pa.Table.to_reader` in PyArrow 8.0.0+, .

@lhoestq @albertvillanova wdyt? We could use such API to efficiently iterate over the batches in `map` before processing them.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4798). All of your documentation changes will be reflected on that endpoint.

This is more efficient since it doesn't bring the data in memory:
```python
for i in range(len(dset) // batch_size)
    start = i * batch_size
    end = min((i+1) * batch_size, len(dset))
    shard = dset.select(range(start, end))
```

@marianna13 can you give more details on when it would be handy to have this shard generator ?

> This is more efficient since it doesn't bring the data in memory:
> 
> ```python
> for i in range(len(dset) // batch_size)
>     start = i * batch_size
>     end = min((i+1) * batch_size, len(dset))
>     shard = dset.select(range(start, end))
> ```
> 
> @marianna13 can you give more details on when it would be handy to have this shard generator ?

Sure! I used such generator when I needed to process a very large dataset  (>1TB) in parallel, I've found out empirically that it's much more efficient to do that by processing only one part of the dataset with the shard generator. I tried to use a map with batching but it causesd oom errors, I tried to use the normal shard and here's what I came up with. So I thought it might be helpful to someone else!

I see thanks ! `map` should work just fine even at this scale, feel free to open an issue if you'd like to discuss your OOM issue.

Regarding `shard_generator`, since it is pretty straightforward to get shards I'm not sure we need that extra Dataset method

Hi again! We've just added `_iter_batches(batch_size)` to the `Dataset` API for fast iteration over batches/chunks, so I think we can close this PR. Compared to this implementation, `_iter_batches` leverages `pa.Table.to_reader` for chunking, which makes it significantly faster.