Add DatasetDict.to_pandas
From discussions in https://github.com/huggingface/datasets/issues/5189, for tabular data it doesn't really make sense to have to do
```python
df = load_dataset(...)["train"].to_pandas()
```
because many datasets are not split.

In this PR I added `to_pandas` to `DatasetDict` which returns the DataFrame:

If there's only one split, you don't need to specify the split name:
```python
df = load_dataset(...).to_pandas()
```
EDIT: and if a dataset has multiple splits:
```python
df = load_dataset(...).to_pandas(splits=["train", "test"])
# or
df = load_dataset(...).to_pandas(splits="all")

# raises an error because you need to select the split(s) to convert
load_dataset(...).to_pandas()
```

I do have one question though @merveenoyan @adrinjalali @mariosasko:

Should we raise an error if there are multiple splits and ask the user to choose one explicitly ?

