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tf.estimator.Head

View source on GitHub

Class Head

Interface for the head/top of a model.

Aliases:

  • Class tf.compat.v1.estimator.Head
  • Class tf.compat.v2.estimator.Head

Head sits on top of the model network and handles computing the outputs of the network. Given logits (or output of a hidden layer), a Head knows how to compute predictions, loss, train_op, metrics and export outputs. It is meant to:

  1. Simplify writing model_fn and to make model_fn more configurable for Estimator.
  2. Simpilfy creating loss and metrics for the train and test loop in Eager execution.
  3. Support wide range of machine learning models. Since most heads can work with logits, they can support DNN, RNN, Wide, Wide&Deep, Global objectives, Gradient boosted trees and many other types of machine learning models.

Common usage:

Here is simplified model_fn to build a DNN regression model.

def _my_dnn_model_fn(features, labels, mode, params, config=None):
  # Optionally your callers can pass head to model_fn as a param.
  head = tf.estimator.RegressionHead(...)

  # TODO(b/117839674): update feature_column
  inputs = tf.feature_column.input_layer(features, ...)

  # Compute logits with tf.keras.layers API
  hidden_layer0 = tf.keras.layers.Dense(
      units=1000, activation="relu")(inputs)
  hidden_layer1 = tf.keras.layers.Dense(
      units=500, activation="relu")(hidden_layer0)
  logits = tf.keras.layers.Dense(
      units=head.logits_dimension, activation=None)(hidden_layer1)

  # Or use Keras model for logits computation
  model = tf.keras.Sequential()
  model.add(tf.keras.layers.Dense(units=1000, activation="relu"))
  model.add(tf.keras.layers.Dense(units=500, activation="relu"))
  model.add(tf.keras.layers.Dense(
     units=head.logits_dimension, activation=None))
  logits = model(inputs)

  return head.create_estimator_spec(
      features=features,
      labels=labels,
      mode=mode,
      logits=logits,
      optimizer=optimizer)

Properties

logits_dimension

Size of the last dimension of the logits Tensor.

Often is the number of classes, labels, or real values to be predicted. Typically, logits is of shape [batch_size, logits_dimension].

Returns:

The expected size of the logits tensor.

loss_reduction

One of tf.losses.Reduction.

Describes how to reduce training loss over batch, such as mean or sum.

Returns:

The type of loss reduction used in the head.

name

The name of this head.

Returns:

A string.

Methods

create_estimator_spec

View source

create_estimator_spec(
    features,
    mode,
    logits,
    labels=None,
    optimizer=None,
    trainable_variables=None,
    train_op_fn=None,
    update_ops=None,
    regularization_losses=None
)

Returns EstimatorSpec that a model_fn can return.

It is recommended to pass all args via name.

Args:

  • features: Input dict mapping string feature names to Tensor or SparseTensor objects containing the values for that feature in a minibatch. Often to be used to fetch example-weight tensor.
  • mode: Estimator's ModeKeys.
  • logits: Logits Tensor to be used by the head.
  • labels: Labels Tensor, or dict mapping string label names to Tensor objects of the label values.
  • optimizer: An tf.keras.optimizers.Optimizer instance to optimize the loss in TRAIN mode. Namely, sets train_op = optimizer.get_updates(loss, trainable_variables), which updates variables to minimize loss.
  • trainable_variables: A list or tuple of Variable objects to update to minimize loss. In Tensorflow 1.x, by default these are the list of variables collected in the graph under the key GraphKeys.TRAINABLE_VARIABLES. As Tensorflow 2.x doesn't have collections and GraphKeys, trainable_variables need to be passed explicitly here.
  • train_op_fn: Function that takes a scalar loss Tensor and returns an op to optimize the model with the loss in TRAIN mode. Used if optimizer is None. Exactly one of train_op_fn and optimizer must be set in TRAIN mode. By default, it is None in other modes. If you want to optimize loss yourself, you can pass lambda _: tf.no_op() and then use EstimatorSpec.loss to compute and apply gradients.
  • update_ops: A list or tuple of update ops to be run at training time. For example, layers such as BatchNormalization create mean and variance update ops that need to be run at training time. In Tensorflow 1.x, these are thrown into an UPDATE_OPS collection. As Tensorflow 2.x doesn't have collections, update_ops need to be passed explicitly here.
  • regularization_losses: A list of additional scalar losses to be added to the training loss, such as regularization losses.

Returns:

EstimatorSpec.

loss

View source

loss(
    labels,
    logits,
    features=None,
    mode=None,
    regularization_losses=None
)

Returns a loss Tensor from provided arguments.

Note that, the args of features and mode are most likely not used, but some Head implementations may require them.

Args:

  • labels: Labels Tensor, or dict mapping string label names to Tensor objects of the label values.
  • logits: Logits Tensor to be used for loss construction.
  • features: Input dict mapping string feature names to Tensor or SparseTensor objects containing the values for that feature in a minibatch. Often to be used to fetch example-weight tensor.
  • mode: Estimator's ModeKeys. To be used in case loss calculation is different in Train and Eval mode.
  • regularization_losses: A list of additional scalar losses to be added to the training loss, such as regularization losses.

Returns:

A scalar Tensor representing regularized training loss used in train and eval.

metrics

View source

metrics(regularization_losses=None)

Returns a dict of metric objects.

Args:

  • regularization_losses: A list of additional scalar losses to be added to the training loss, such as regularization losses.

Returns:

A dict of metrics keyed by string name. The value is an instance of Metric class.

predictions

View source

predictions(
    logits,
    keys=None
)

Returns a dict of predictions from provided logits.

Args:

  • logits: Logits Tensor to be used for prediction construction.
  • keys: A list of string for prediction keys. Defaults to None, meaning if not specified, predictions will be created for all the pre-defined valid keys in the head.

Returns:

A dict of predicted Tensor keyed by prediction name.

update_metrics

View source

update_metrics(
    eval_metrics,
    features,
    logits,
    labels,
    mode=None,
    regularization_losses=None
)

Updates metric objects and returns a dict of the updated metrics.

Args:

  • eval_metrics: A dict of metrics to be updated.
  • features: Input dict mapping string feature names to Tensor or SparseTensor objects containing the values for that feature in a minibatch. Often to be used to fetch example-weight tensor.
  • logits: logits Tensor to be used for metrics update.
  • labels: Labels Tensor, or dict mapping string label names to Tensor objects of the label values.
  • mode: Estimator's ModeKeys.
  • regularization_losses: A list of additional scalar losses to be added to the training and evaluation loss, such as regularization losses.

Returns:

A dict of updated metrics keyed by name. The value is an instance of Metric class.