Allow the user to specify only off-topic entries.

[JosephCatrambone]

Prior to this change there was no way for a user to specify only a set of topics that was off limit. There needed to be at least one item on topic. With these changes, a user can now specify only restricted topics.

The pass criteria are now: no off-topic matters AND (at least one on-topic matter IF they exist).

Example:

r = RestrictToTopic(invalid_topics=["tacos",])

r.validate("I went to the grocery store on Thursday.")
PassResult(outcome='pass', ...)

r.validate("I decide to buy some Taco Bell.")
FailResult(outcome='fail', error_message="Invalid topics found: ['tacos']", ...)

r = RestrictToTopic()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
----> 1 r = RestrictToTopic()

File ~/Source/restricttotopic/validator/main.py:118, in RestrictToTopic.__init__(self, valid_topics, invalid_topics, device, model, llm_callable, disable_classifier, classifier_api_endpoint, disable_llm, on_fail, zero_shot_threshold, llm_threshold, **kwargs)
    115     self._invalid_topics = invalid_topics
    117 if not (self._valid_topics or self._invalid_topics):
--> 118     raise ValueError("Either valid topics or invalid topics must be specified.")
    120 self._device = (
    121     str(device).lower()
    122     if str(device).lower() in ["cpu", "mps"]
    123     else int(device)
    124 )
    125 self._model = model

ValueError: Either valid topics or invalid topics must be specified.

r = RestrictToTopic(valid_topics=["tacos",])

r.validate("I decide to buy some Taco Bell.")
PassResult(outcome='pass', value_override=<class 'guardrails.classes.validation.validation_result.PassResult.ValueOverrideSentinel'>, metadata=None, validated_chunk=None)