tf.estimator.Head

Interface for the head/top of a model.

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(...)

  feature_columns = tf.feature_column.numeric_column(...)
  feature_layer = tf.keras.layers.DenseFeatures(feature_columns)
  inputs = feature_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)

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].

loss_reduction One of tf.losses.Reduction.

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

name The name of this head.

Methods

create_estimator_spec

View source

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

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

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

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

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. In most cases, this arg is not used and can be removed in the method implementation.
regularization_losses A list of additional scalar losses to be added to the training and evaluation loss, such as regularization losses. Note that, the mode arg is not used in the tf.estimator.*Head. If the update of the metrics doesn't rely on mode, it can be safely ignored in the method signature.