Log a similarity evaluation result dictionary as TSV to logfile.

def log_similarity_result(logfile, result):
    """Log a similarity evaluation result dictionary as TSV to logfile."""
    assert result['task'] == 'similarity'

    if not logfile:
        return

    with open(logfile, 'a') as f:
        f.write('\t'.join([
            str(result['global_step']),
            result['task'],
            result['dataset_name'],
            json.dumps(result['dataset_kwargs']),
            result['similarity_function'],
            str(result['spearmanr']),
            str(result['num_dropped']),
        ]))

        f.write('\n')