pretty print dataset objects
Great, as you found it useful I improved the code a bit to automate indentation in the parent class, so that the child repr doesn't need to guess the indentation level, while repr'ing nicely on its own.

- do we want indent=4 or 2?
- do we want `{` ... `}` or w/o?

currently it's indent4 and w/ curly braces, so it looks:

```
DatasetDict({
    train: Dataset({
        features: ['text', 'headline', 'title'],
        num_rows: 157252
    })
    validation: Dataset({
        features: ['text', 'headline', 'title'],
        num_rows: 5599
    })
    test: Dataset({
        features: ['text', 'headline', 'title'],
        num_rows: 5577
    })
})
```
just child:
```
Dataset({
    features: ['text', 'headline', 'title'],
    num_rows: 5577
})
```

 Yes!  A lot better indeed!
Currently, if I do:
```
from datasets import load_dataset
load_dataset("wikihow", 'all', data_dir="/hf/pegasus-datasets/wikihow/")
```
I get:
```

DatasetDict({'train': Dataset(features: {'text': Value(dtype='string', id=None),
'headline': Value(dtype='string', id=None), 'title': Value(dtype='string',
id=None)}, num_rows: 157252), 'validation': Dataset(features: {'text':
Value(dtype='string', id=None), 'headline': Value(dtype='string', id=None),
'title': Value(dtype='string', id=None)}, num_rows: 5599), 'test':
Dataset(features: {'text': Value(dtype='string', id=None), 'headline':
Value(dtype='string', id=None), 'title': Value(dtype='string', id=None)},
num_rows: 5577)})
```

This is not very readable. 

Can we either have a better `__repr__` or have a custom method to nicely pprint the dataset object? 

Here is my very simple attempt. With this PR, it produces:
```
DatasetDict({
  train:   Dataset({
    features: ['text', 'headline', 'title'],
    num_rows: 157252
  })
  validation:   Dataset({
    features: ['text', 'headline', 'title'],
    num_rows: 5599
  })
  test:   Dataset({
    features: ['text', 'headline', 'title'],
    num_rows: 5577
  })
})
```
I did omit the data types on purpose to make it more readable, but it shouldn't be too difficult to integrate those too.

note that this PR also fixes the inconsistency in output that in master misses enclosing `{}` for Dataset, but it is there for `DatasetDict` - or perhaps it was by design.

I'm totally not attached to this format, just wanting something more readable. One approach could be to serialize to `json.dumps` or something similar. It'd make the indentation simpler.

Thank you.