Add tests for no_trainer + fixes
# New tests for the `no_trainer` scripts

## What does this add?

- Adds in test cases for each of the `no_trainer` scripts, mocking how the Transformers counterparts work
- Fixes a small variety of bugs inside the `no_trainer` scripts, discovered while writing these tests
- Introduces the ability to write a json file at the end of training, so that tests can be performed, similar to the Transformers tests