## Class Head

Interface for the head/top of a model.

THIS CLASS IS DEPRECATED. See contrib/learn/README.md for general migration instructions.

Given logits (or output of a hidden layer), a Head knows how to compute predictions, loss, default metric and export signature. It is meant to,

1) Simplify writing model_fn and to make model_fn more configurable 2) 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. 2) To allow users to seamlessly switch between 1 to n heads for multi objective learning (See _MultiHead implementation for more details)

Common usage: Here is simplified model_fn to build a multiclass DNN model.

def _my_dnn_model_fn(features, labels, mode, params, config=None):
# Optionally your callers can pass head to model_fn as a param.
input = tf.contrib.layers.input_from_feature_columns(features, ...)
last_hidden_layer_out = tf.contrib.layers.stack(
input, tf.contrib.layers.fully_connected, [1000, 500])
logits = tf.contrib.layers.fully_connected(

def _train_op_fn(loss):
return optimizer.minimize(loss)

features=features,
labels=labels,
mode=mode,
train_op_fn=_train_op_fn,
logits=logits,
scope=...)


Most heads also support logits_input which is typically the output of the last hidden layer. Some heads (like heads responsible for candidate sampling or hierarchical softmax) intrinsically will not support logits and you have to pass logits_input. Here is a common usage,

return head.create_model_fn_ops(
features=features,
labels=labels,
mode=mode,
train_op_fn=_train_op_fn,
logits_input=last_hidden_layer_out,
scope=...)


There are cases where computing and applying gradients can not be meaningfully captured with train_op_fn we support (for example, with sync optimizer). In such case, you can take the responsibility on your own. Here is a common use case,

model_fn_ops = head.create_model_fn_ops(
features=features,
labels=labels,
mode=mode,
train_op_fn=tf.contrib.learn.no_op_train_fn,
logits=logits,
scope=...)
if mode == tf.contrib.learn.ModeKeys.TRAIN:
optimizer = ...
sync = tf.train.SyncReplicasOptimizer(opt=optimizer, ...)
update_op = tf.contrib.layers.optimize_loss(optimizer=sync,
loss=model_fn_ops.loss, ...)
hooks = [sync.make_session_run_hook(is_chief)]
... update train_op and hooks in ModelFnOps and return


## Properties

### logits_dimension

Size of the last dimension of the logits Tensor.

Typically, logits is of shape [batch_size, logits_dimension].

#### Returns:

The expected size of the logits tensor.

## Methods

### create_model_fn_ops

create_model_fn_ops(
features,
mode,
labels=None,
train_op_fn=None,
logits=None,
logits_input=None,
scope=None
)


Returns ModelFnOps that a model_fn can return.

Please note that, + Exactly one of logits and logits_input must be provided. + All args must be passed via name.

#### Args:

• features: Input dict of Tensor objects.
• mode: Estimator's ModeKeys.
• labels: Labels Tensor, or dict of same.
• train_op_fn: Function that takes a scalar loss Tensor and returns an op to optimize the model with the loss. This is used in TRAIN mode and must not be None. None is allowed in other modes. If you want to optimize loss yourself you can pass no_op_train_fn and then use ModeFnOps.loss to compute and apply gradients.
• logits: logits Tensor to be used by the head.
• logits_input: Tensor from which to build logits, often needed when you don't want to compute the logits. Typically this is the activation of the last hidden layer in a DNN. Some heads (like the ones responsible for candidate sampling) intrinsically avoid computing full logits and only accepts logits_input.
• scope: Optional scope for variable_scope.

#### Returns:

An instance of ModelFnOps.

#### Raises:

• ValueError: If mode is not recognized.
• ValueError: If neither or both of logits and logits_input is provided.