Decodes `targets` given encoded source. Args: theta: A `.NestedMap` object containing weights' values of this layer and its children layers. encoder_outputs: a NestedMap computed by encoder. Expected to contain: encoded - source encoding, of shape [time, batch, depth
(self,
theta,
encoder_outputs,
targets,
interpolation_batch=None,
lambdas=None)
source not stored for this graph (policy: none)