Change checksum field to tokenizer_checksum

[PeanutButterRat]

What does this PR do? Please describe:
This PR fixes a small bug with text_tokenizer.py where it would pull the tokenizer checksum from the checksum field in the asset card which is instead used for the model checksum. Now, models and tokenizers don't share the same checksum field and can therefore be validated separately as expected.

Fixes #865

Does your PR introduce any breaking changes? If yes, please list them:
None

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)