Featurization

In order to apply machine learning algorithms to conversational AI, we need to build up vector representations of conversations.

Each story corresponds to a tracker which consists of the states of the conversation just before each action was taken.

State Featurizers

Every event in a trackers history creates a new state (e.g. running a bot action, receiving a user message, setting slots). Featurizing a single state of the tracker has a couple steps:

  1. Tracker provides a bag of active features:
    • features indicating intents and entities, if this is the first state in a turn, e.g. it’s the first action we will take after parsing the user’s message. (e.g. [intent_restaurant_search, entity_cuisine] )
    • features indicating which slots are currently defined, e.g. slot_location if the user previously mentioned the area they’re searching for restaurants.
    • features indicating the results of any API calls stored in slots, e.g. slot_matches
    • features indicating what the last action was (e.g. prev_action_listen)
  2. Convert all the features into numeric vectors:

    We use the X, y notation that’s common for supervised learning, where X is an array of shape (num_data_points, time_dimension, num_input_features), and y is an array of shape (num_data_points, num_bot_features) or (num_data_points, time_dimension, num_bot_features) containing the target class labels encoded as one-hot vectors.

    The target labels correspond to actions taken by the bot. To convert the features into vector format, there are different feaurizers available:

    • BinarySingleStateFeaturizer creates a binary one-hot encoding:

      The vectors X, y indicate a presence of a certain intent, entity, previous action or slot e.g. [0 0 1 0 0 1 ...].

    • LabelTokenizerSingleStateFeaturizer creates an vector based on the feature label:

      All active feature labels (e.g. prev_action_listen) are split into tokens and represented as a bag-of-words. For example, actions utter_explain_details_hotel and utter_explain_details_restaurant will have 3 features in common, and differ by a single feature indicating a domain.

      Labels for user inputs (intents, entities) and bot actions are featurized separately. Each label in the two categories is tokenized on a special character split_symbol (e.g. action_search_restaurant = {action, search, restaurant}), creating two vocabularies. A bag-of-words representation is then created for each label using the appropriate vocabulary. The slots are featurized as binary vectors, indicating their presence or absence at each step of the dialogue.

Note

If the domain defines the possible actions, [ActionGreet, ActionGoodbye], two additional default actions are added: [ActionListen, ActionRestart]. Therefore, label 0 indicates default action listen, label 1 default restart, label 2 a greeting and 3 indicates goodbye.

Tracker Featurizers

It’s often useful to include a bit more history than just the current state when predicting an action. The TrackerFeaturizer iterates over tracker states and calls a SingleStateFeaturizer for each state. There are two different tracker featurizers:

1. Full Dialogue

FullDialogueTrackerFeaturizer creates numerical representation of stories to feed to a recurrent neural network where the whole dialogue is fed to a network and the gradient is backpropagated from all time steps. Therefore, X is an array of shape (num_stories, max_dialogue_length, num_input_features) and y is an array of shape (num_stories, max_dialogue_length, num_bot_features). The smaller dialogues are padded with -1 for all features, indicating no values for a policy.

2. Max History

MaxHistoryTrackerFeaturizer creates an array of previous tracker states for each bot action or utterance, with the parameter max_history defining how many states go into each row in X. Deduplication is performed to filter out duplicated turns (bot actions or bot utterances) in terms of their previous states. Hence X has shape (num_unique_turns, max_history, num_input_features) and y is an array of shape (num_unique_turns, num_bot_features).

For some algorithms a flat feature vector is needed, so X should be reshaped to (num_unique_turns, max_history * num_input_features). If numeric target class labels are needed instead of one-hot vectors, use y.argmax(axis=-1).