Computes cross-entropy loss given the softmax input, labels and weights. Args: theta: A `.NestedMap` object containing weights' values of this layer and its children layers. softmax_input: A tensor of shape [time, batch, p.softmax.input_dim]. target_labels: A matrix of
(self,
theta,
softmax_input,
target_labels,
target_weights,
target_paddings,
target_segment_ids=None,
time_axis=0)
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