readme: Test metrics:
  
  {'eval_loss': 2.040231943130493, 'eval_rouge1': 80.3408, 'eval_rouge2': 72.5633, 'eval_rougeL': 79.3605, 'eval_rougeLsum': 79.3773, 'eval_bleu-4': 69.3542, 'eval_meteor': 79.3628, 'eval_gen_len': 15.7678, 'eval_runtime': 576.1978, 'eval_samples_per_second': 18.709, 'eval_steps_per_second': 1.17, 'epoch': 5.87}
  