Computes the log-likelihood of tag sequences in a CRF.

inputs A [batch_size, max_seq_len, num_tags] tensor of unary potentials to use as input to the CRF layer.
tag_indices A [batch_size, max_seq_len] matrix of tag indices for which we compute the log-likelihood.
sequence_lengths A [batch_size] vector of true sequence lengths.
transition_params A [num_tags, num_tags] transition matrix, if available.

log_likelihood A [batch_size] Tensor containing the log-likelihood of each example, given the sequence of tag indices.
transition_params A [num_tags, num_tags] transition matrix. This is either provided by the caller or created in this function.