Creating custom dataset results in error while calling the map() function
Hi,

the `_data` attribute is missing due to `MyDataset.__init__` not calling the parent `__init__`. However, I don't think it's a good idea to subclass the `datasets.Dataset` class (e.g. it's kind of dangerous to override `datasets.Dataset.__getitem__`). Instead, it's better to follow the "association over inheritance" approach with a simple wrapper class that delegates calls to a wrapped `Dataset` (map, etc.). Btw, the library offers the `datasets.Dataset.from_pandas` class method to directly create a `datasets.Dataset` from the dataframe.
calling `map()` of `datasets` library results into an error while defining a Custom dataset.
Reproducible example:
```
import datasets
class MyDataset(datasets.Dataset):

    def __init__(self, sentences):
        "Initialization"
        self.samples = sentences

    def __len__(self):
        "Denotes the total number of samples"
        return len(self.samples)

    def __getitem__(self, index):
        "Generates one sample of data"
        # Select sample
        # Load data and get label
        samples = self.samples[index]

        return samples

def preprocess_function_train(examples):
        inputs = examples
        labels = [example+tokenizer.eos_token for example in examples ]
        inputs = tokenizer(inputs, max_length=30, padding=True, truncation=True)
        labels = tokenizer(labels, max_length=30, padding=True, truncation=True)
        model_inputs = inputs
        model_inputs["labels"] = labels["input_ids"]
        print("about to return")
        return model_inputs


##train["sentence"] is dataframe column
train_dataset = MyDataset(train['sentence'].values.tolist())
train_dataset = train_dataset.map(
            preprocess_function,
            batched = True,
            batch_size=32
        )
```

Stack trace of error:
```
Traceback (most recent call last):
  File "dir/train_generate.py", line 362, in <module>
    main()
  File "dir/train_generate.py", line 245, in main
    train_dataset = train_dataset.map(
  File "anaconda_dir/anaconda3/envs/env1/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 1244, in map
    return self._map_single(
  File "anaconda_dir/anaconda3/envs/env1/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 149, in wrapper
    unformatted_columns = set(self.column_names) - set(self._format_columns or [])
  File "anaconda_dir/anaconda3/envs/env1/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 526, in column_names
    return self._data.column_names
AttributeError: 'MyDataset' object has no attribute '_data'
```