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!