title: Can't understand about pytorch tensor broadcast
body: <p>I have the following code:</p>
  <pre><code>import torch
  d = 2
  n = 50
  X = torch.randn(n,d)
  z = torch.tensor([[-1.0], [2.0]])
  y = X @ z
  X.size()
  z.size()
  y.size()
  </code></pre>
  <p>The output is:</p>
  <pre><code>torch.Size([50, 2])
  torch.Size([2, 1])
  torch.Size([50, 1])
  </code></pre>
  <p>My question is, why after broadcasting, the size of the result <code>y</code> is <code>[50,1]</code> rather than <code>[50,2]</code>, I think it should be <code>[50,2]</code>, am I correct?</p>