Delay resetting IntentSlotClassificationModel restore state until after the bert model is initialized
Fixes #2686 

When a trained IntentSlotClassificationModel is exported via `save_to` the resulting archive includes BERT configuration artifacts. During `IntentSlotClassificationModel.restore_from` using the saved archived the BERT model should be initialized using these configuration artifacts. However, because the model restore state is reset before initializing the BERT model these artifacts cannot be found.
Delaying the resetting of the restore state until after the BERT model is initialized resolves the problem.