ValueError when rename_column on splitted dataset
Hi,

This is a bug so thanks for reporting it. `Dataset.__setstate__`  is the problem, which is called when `Dataset.rename_column` tries to copy the dataset with `copy.deepcopy(self)`. This only happens if the `split` arg in `load_dataset` was defined as `ReadInstruction`.

To overcome this issue, use the named splits API (for now):
```python
train_ds, test_ds = load_dataset(
    path='csv',               
    delimiter='\t',          
    data_files=text_files,    
    split=['train[:90%]', 'train[-10%:]'],
)

train_ds = train_ds.rename_column('sentence', 'text')
``` This has been fixed in #2043 , thanks @mariosasko 
The fix is available on master and we'll do a new release soon :)

feel free to re-open if you still have issues
Hi there,
I am loading `.tsv` file via `load_dataset` and subsequently split the rows into training and test set via the `ReadInstruction` API like so:

```python
split = {
    'train': ReadInstruction('train', to=90, unit='%'),
    'test': ReadInstruction('train', from_=-10, unit='%')
}

dataset = load_dataset(
    path='csv',               # use 'text' loading script to load from local txt-files
    delimiter='\t',           # xxx
    data_files=text_files,    # list of paths to local text files
    split=split,              # xxx
)

dataset
```

Part of output:
```python
DatasetDict({
    train: Dataset({
        features: ['sentence', 'sentiment'],
        num_rows: 900
    })
    test: Dataset({
        features: ['sentence', 'sentiment'],
        num_rows: 100
    })
})
```
Afterwards I'd like to rename the 'sentence' column to 'text' in order to be compatible with my modelin pipeline. If I run the following code I experience a `ValueError` however:
```python
dataset['train'].rename_column('sentence', 'text')
```
```python
/usr/local/lib/python3.7/dist-packages/datasets/splits.py in __init__(self, name)
    353         for split_name in split_names_from_instruction:
    354             if not re.match(_split_re, split_name):
--> 355                 raise ValueError(f"Split name should match '{_split_re}'' but got '{split_name}'.")
    356 
    357     def __str__(self):

ValueError: Split name should match '^\w+(\.\w+)*$'' but got 'ReadInstruction('.
```
In particular, these behavior does not arise if I use the deprecated `rename_column_` method. Any idea what causes the error? Would assume something in the way I defined the split.

Thanks in advance! :)