In 'best_point' don't require GeneratorRun to have best_arm_predictions to predict from model

[esantorella]

Summary: Context:

get_best_parameters_from_model_predictions_with_trial_index will only predict from a model if there are best_arm_predictions on the GeneratorRun. This doesn't make sense, since it's about to construct and fit a new model and use it to generate predicts. Any existing best_arm_predictions are not used.

This PR:

• Removes the gr.best_arm_predictions is not None check
• Changes how some imported functions are referenced in best_point_mixin.py (doesn't change functionality)

Reviewed By: mpolson64

Differential Revision: D62594017

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D62594017

[facebook-github-bot]

This pull request has been merged in facebook/Ax@2fc80f11ca7df8b192eaca0590e0902a5196b8e6.