Skip dataset verifications by default
Skip the dataset verifications (split and checksum verifications, duplicate keys check) by default unless a dataset is being tested (`datasets-cli test/run_beam`). The main goal is to avoid running the checksum check in the default case due to how expensive it can be for large datasets.

PS: Maybe we should deprecate `ignore_verifications`, which is `True` now by default, and give it a different name?