title: what does the error "Length of label is not same with #data" when I call lightgbm.train
body: <p>I'm pretty new the LightGBM, and when I try to apply lightgbm.train on my dataset, I got this error:</p>
  
  <p><code>LightGBMError: Length of label is not same with #data</code></p>
  
  <p>I'm not sure where I made a mistake. I tried</p>
  
  <p><code>model = lightgbm.train(params, train_data, valid_sets=test_data, early_stopping_rounds=150, verbose_eval=200)</code></p>
  
  <p>Thanks in advance.</p>