Gating model config improvements

[steffencruz]

Custom gating model flag is independent of gating.model_name, so people can run custom gating models without the custom flag which at a minimum disrupts the wandb tagging.

• The custom flag should be triggered based on whether gating.model_name == __DEFAULT_GATING_MODEL_NAME__ or something similar.
• Resync should be pushed down to the base class as it's identical in both subclasses
• Logic of creating SentenceEmbedGatingModel vs GatingModel is also brittle. We default to GatingModel for any other gating.model_name if the custom flag is not set, which will almost certainly break with other models.