title: what does padding_idx do in nn.embeddings()
body: <p>I'm learning pytorch and
  I'm wondering what does the <code>padding_idx</code> attribute do in <code>torch.nn.Embedding(n1, d1, padding_idx=0)</code>?
  I have looked everywhere and couldn't find something I can get. 
  Can you show example to illustrate this?</p>