Updates to SignatureOptimizer [bug fixes + track stats]

[klopsahlong]

In this PR, we make a few small updates to the SignatureOptimizer teleprompter:

1. We fix a bug in the previous implementation, where the prompt_model defaulted to a string, which threw an error. We now default prompt_model to None, and use the default lm as our prompt model when this is the case.
2. We fix a bug where duplicate prompt candidates were being included in the prompt history.
3. We add in the ability to track statistics of the quality of prompts found over each iteration of the SignatureOptimizer algorithm with the field track_stats