Add docstrings to type validator functions

[dhruv7477]

Add missing docstrings to the 11 undocumented public functions in src/transformers/utils/type_validators.py. The existing interval, probability, and activation_fn_key functions already had docstrings; this brings the rest of the file up to the same standard.

Each docstring follows the Google-style format used elsewhere in the file, listing accepted types and valid values where non-obvious (e.g. the valid keys for image_size_validator, the accepted device names for device_validator, the nested structure accepted by video_metadata_validator).

No logic changes.

[Rocketknight1]

We generally don't want things like code agent docstrings right now, I'm sorry! We might do this ourselves in an agent cleanup pass at some point, but it just creates a lot of reviewer noise at the moment.