Clean up pipeline vs model and previous PR

[gabegma]

Description:

• Cleanup following the previous PRs.
• For model vs pipeline, I tried to remain as coherent as possible. I refer model as the callable function in PipelineDefinition, even though this model can be a HF pipeline, a model, a custom pipeline, or a file prediction reader.

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge it.

• ISSUE NUMBER. You linked the issue number (Ex: Resolve #XXX).
• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[gabegma]

This is no longer true, but the tokenizer still has is_fast=False in this case. I removed this line in the test since it wasn't testing something useful.

[JosephMarinier]

👍