readme: Test metrics:
  
  {'eval_loss': 2.046710968017578, 'eval_rouge1': 79.8767, 'eval_rouge2': 72.035, 'eval_rougeL': 78.8221, 'eval_rougeLsum': 78.8505, 'eval_bleu-4': 68.8372, 'eval_meteor': 78.8501, 'eval_gen_len': 15.6601, 'eval_runtime': 590.1399, 'eval_samples_per_second': 18.267, 'eval_steps_per_second': 1.142, 'epoch': 5.87}
  