AutoMLSearch: update objective string to use human-readable format

• The get_objective(objective) only works if the input is an Objective or a snakecase representation.
• It does not work if the objective str is something like log loss binary or Log Loss Binary, even those this is from the name of the Objective

[dsherry]

@freddyaboulton ah good point about that…

I suggest we don’t add a flag for that to get_objective. I think we should put validation logic into AutoMLSearch instead, where we read in and parse the objective.

        if objective == 'auto':
            objective = self._DEFAULT_OBJECTIVES[self.problem_type.value]
        objective_obj = get_objective(objective)
        self._validate_objective(objective_obj)
        self.objective = objective

If we do that validation at a lower level I think its too confusing.

[freddyaboulton]

@dsherry I can take this on! I like the idea of deleting Options in favor of get_importable_subclasses. I think we would need to pass in an allowed_in_automl_search flag to get_objective to keep the current behavior of not allowing all objectives to be used in AutoMLSearch (unless we want to get rid of that restriction).