SentencePieceProcessor: piece_to_id throws when piece token is unknown

[shadeMe]

Description

Previously, we had the inconsistent behavior where piece_to_id would return the ID of the unk metatoken when the piece token was out-of-vocab, but id_to_piece would throw when an invalid piece ID was passed to it. This PR fixes that by making piece_to_id throw as well.

Types of change

Refactor/enhancement

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[shadeMe]

Closed after internal discussion - id_to_piece and piece_to_id will return Optionals across the board instead of throwing.