Support sliced list arrays in cast
{'login': 'lhoestq', 'id': 42851186, 'node_id': 'MDQ6VXNlcjQyODUxMTg2', 'avatar_url': 'https://avatars.githubusercontent.com/u/42851186?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lhoestq', 'html_url': 'https://github.com/lhoestq', 'followers_url': 'https://api.github.com/users/lhoestq/followers', 'following_url': 'https://api.github.com/users/lhoestq/following{/other_user}', 'gists_url': 'https://api.github.com/users/lhoestq/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lhoestq/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lhoestq/subscriptions', 'organizations_url': 'https://api.github.com/users/lhoestq/orgs', 'repos_url': 'https://api.github.com/users/lhoestq/repos', 'events_url': 'https://api.github.com/users/lhoestq/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lhoestq/received_events', 'type': 'User', 'site_admin': False}
[]
[]
There is this issue in pyarrow:
```python
import pyarrow as pa

arr = pa.array([[i * 10] for i in range(4)])
arr.cast(pa.list_(pa.int32()))  # works

arr = arr.slice(1)
arr.cast(pa.list_(pa.int32()))  # fails
# ArrowNotImplementedError("Casting sliced lists (non-zero offset) not yet implemented")
```

However in `Dataset.cast` we slice tables to cast their types (it's memory intensive), so we have the same issue.
Because of this it is currently not possible to cast a Dataset with a Sequence feature type (unless the table is small enough to not be sliced).

In this PR I fixed this by resetting the offset of `pyarrow.ListArray` arrays to zero in the table before casting.
I used `pyarrow.compute.subtract` function to update the offsets of the ListArray.

cc @abhi1thakur @SBrandeis 
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/2461/reactions', 'total_count': 1, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 1, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/2461', 'html_url': 'https://github.com/huggingface/datasets/pull/2461', 'diff_url': 'https://github.com/huggingface/datasets/pull/2461.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2461.patch', 'merged_at': '2021-06-08T17:56:23Z'}