Computes forward decoding in a linear-chain CRF.
crf_decode_forward(inputs, state, transition_params, sequence_lengths)
A [batch_size, num_tags] matrix of unary potentials.
A [batch_size, num_tags] matrix containing the previous step's score values.
A [num_tags, num_tags] matrix of binary potentials.
A [batch_size] vector of true sequence lengths.
backpointers: A [batch_size, num_tags] matrix of backpointers. new_state: A [batch_size, num_tags] matrix of new score values.