title: How to set n_batches_per_layer in tf.estimator.BoostedTreesClassifier
body: <p>How should I think when setting the n_batches_per_layer?
  I currently select 10 from [1, 10, 100] as a result of hyperparameter tuning.</p>
  <p>But when I thought about it. If I divide my data by the batch_size, I get 6.8 batches.
  So I'm thinking that perhaps 7 would be better, that would cover all the data, with minimal overlap.</p>
  <p>I guess my question boils down to, what does this parameter actually do?</p>