Add unit tests to candle-transformers

As noted by @ivarflakstad in #3264, candle-transformers could benefit from more unit tests.

> Not opposed to adding unit tests to candle-transformers later. We don't have enough of them in general.

This issue tracks the work to add comprehensive unit tests for candle-transformers models.

## Suggested starting points
- Mamba2 core algorithms (from #3264)
- Other model implementations

## Related
- #3264 (Mamba2 implementation)