'Returns a sample to be used to predict a character during training.
This function is used as a loop_function for an RNN decoder.
Args:
prev: output tensor from previous step of the RNN. A tensor with shape:
[batch_size, num_char_classes].
i: index of a character in the output sequence.
Returns:
A tensor with shape [batch_size, ?] - depth depends on implementation
details.'

@abc.abstractmethod
def get_train_input(self, prev, i):

    pass
