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Computes the alpha values in a linear-chain CRF.

See for reference.

inputs A [batch_size, num_tags] matrix of unary potentials.
state A [batch_size, num_tags] matrix containing the previous alpha values.
transition_params A [num_tags, num_tags] matrix of binary potentials. This matrix is expanded into a [1, num_tags, num_tags] in preparation for the broadcast summation occurring within the cell.
sequence_lengths A [batch_size] vector of true sequence lengths.

new_alphas A [batch_size, num_tags] matrix containing the new alpha values.