View source on GitHub 
A factorized retrieval task.
Inherits From: Task
tfrs.tasks.Retrieval(
loss: Optional[tf.keras.losses.Loss] = None,
metrics: Optional[tfrs.metrics.FactorizedTopK
] = None,
batch_metrics: Optional[List[tf.keras.metrics.Metric]] = None,
loss_metrics: Optional[List[tf.keras.metrics.Metric]] = None,
temperature: Optional[float] = None,
num_hard_negatives: Optional[int] = None,
remove_accidental_hits: bool = False,
name: Optional[Text] = None
) > None
Used in the notebooks
Used in the tutorials 

Recommender systems are often composed of two components:
 a retrieval model, retrieving O(thousands) candidates from a corpus of O(millions) candidates.
 a ranker model, scoring the candidates retrieved by the retrieval model to return a ranked shortlist of a few dozen candidates.
This task defines models that facilitate efficient retrieval of candidates from large corpora by maintaining a twotower, factorized structure: separate query and candidate representation towers, joined at the top via a lightweight scoring function.
Args  

loss

Loss function. Defaults to
tf.keras.losses.CategoricalCrossentropy .

metrics

Object for evaluating topK metrics over a
corpus of candidates. These metrics measure how good the model is at
picking the true candidate out of all possible candidates in the system.
Note, because the metrics range over the entire candidate set, they are
usually much slower to compute. Consider setting compute_metrics=False
during training to save the time in computing the metrics.

batch_metrics

Metrics measuring how good the model is at picking out the true candidate for a query from other candidates in the batch. For example, a batch AUC metric would measure the probability that the true candidate is scored higher than the other candidates in the batch. 
loss_metrics

List of Keras metrics used to summarize the loss. 
temperature

Temperature of the softmax. 
num_hard_negatives

If positive, the num_hard_negatives negative
examples with largest logits are kept when computing crossentropy loss.
If larger than batch size or nonpositive, all the negative examples are
kept.

remove_accidental_hits

When given enables removing accidental hits of examples used as negatives. An accidental hit is defined as a candidate that is used as an inbatch negative but has the same id with the positive candidate. 
name

Optional task name. 
Attributes  

factorized_metrics

The metrics object used to compute retrieval metrics. 
Methods
call
call(
query_embeddings: tf.Tensor,
candidate_embeddings: tf.Tensor,
sample_weight: Optional[tf.Tensor] = None,
candidate_sampling_probability: Optional[tf.Tensor] = None,
candidate_ids: Optional[tf.Tensor] = None,
compute_metrics: bool = True,
compute_batch_metrics: bool = True
) > tf.Tensor
Computes the task loss and metrics.
The main argument are pairs of query and candidate embeddings: the first row of query_embeddings denotes a query for which the candidate from the first row of candidate embeddings was selected by the user.
The task will try to maximize the affinity of these query, candidate pairs while minimizing the affinity between the query and candidates belonging to other queries in the batch.
Args  

query_embeddings

[num_queries, embedding_dim] tensor of query representations. 
candidate_embeddings

[num_queries, embedding_dim] tensor of candidate representations. 
sample_weight

[num_queries] tensor of sample weights. 
candidate_sampling_probability

Optional tensor of candidate sampling probabilities. When given will be be used to correct the logits to reflect the sampling probability of negative candidates. 
candidate_ids

Optional tensor containing candidate ids. When given, factorized topK evaluation will be idbased rather than scorebased. 
compute_metrics

Whether to compute metrics. Set this to False during training for faster training. 
compute_batch_metrics

Whether to compute batch level metrics. Inbatch loss_metrics will still be computed. 
Returns  

loss

Tensor of loss values. 