Adds a new end-to-end test for Mistral 7b

[shralex]

The changes include:

• A notebook in scratch_pad that runs the model from HF, for sample inputs and generates logits, which it stores in a file.
• A test that downloads the HF checkpoint, converts it to MaxText-compatible format, and uses it to do one forward pass on a sample input.
• Compares the logits obtained in both steps above to make sure they're equal.

The old test, which asserted on the generated text being equal, rather than logits, is removed.