Computes recall@k of the predictions with respect to sparse labels.

If class_id is specified, we calculate recall by considering only the entries in the batch for which class_id is in the label, and computing the fraction of them for which class_id is in the top-k predictions. If class_id is not specified, we'll calculate recall as how often on average a class among the labels of a batch entry is in the top-k predictions.

sparse_recall_at_k creates two local variables, true_positive_at_<k> and false_negative_at_<k>, that are used to compute the recall_at_k frequency. This frequency is ultimately returned as recall_at_<k>: an idempotent operation that s