Rename LargeList.dtype to LargeList.feature
Rename `LargeList.dtype` to `LargeList.feature`.

Note that `dtype` is usually used for NumPy data types ("int64", "float32",...): see `Value.dtype`.

However, `LargeList` attribute (like `Sequence.feature`) expects a `FeatureType` instead.

With this renaming:
- we avoid confusion about the expected type and
- we also align `LargeList` with `Sequence`.