title: understanding tensorflow sequence_loss parameters
body: <p>The <code>sequence_Loss</code> module's <a href="https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/ops/seq2seq.py#L1075" rel="noreferrer">source_code</a> has three parameters that are required they list them as outputs, targets, and weights. </p>
  
  <p>Outputs and targets are self explanatory, but I'm looking to better understand is what is the <code>weight</code> parameter? </p>
  
  <p>The other thing I find confusing is that it states that the <code>targets</code> should be the same <code>length</code> as the outputs, what exactly do they mean by the length of a tensor? Especially if its a 3 dimensional tensor. </p>