title: how to get specific elements in tensor (pytorch)
body: <p>I have a tensor, like:
  input:</p>
  <pre><code>a = torch.rand(2,3,5)
  </code></pre>
  <p>output:</p>
  <pre><code>tensor([[[0.2764, 0.2209, 0.8007, 0.1246, 0.4302],
       [0.9716, 0.8063, 0.3904, 0.7574, 0.2392],
       [0.3366, 0.4209, 0.0527, 0.1328, 0.0441]],
  
      [[0.8166, 0.6519, 0.5450, 0.3072, 0.2716],
       [0.0583, 0.0613, 0.8984, 0.0110, 0.4744],
       [0.2269, 0.2693, 0.6447, 0.6078, 0.6148]]])
  </code></pre>
  <p>how can I get :</p>
  <pre><code>tensor([[[0.2764, 0.2209, 0.8007, 0.1246, 0.4302],
   [0.9716, 0.8063, 0.3904, 0.7574, 0.2392]],
  
  [[0.8166, 0.6519, 0.5450, 0.3072, 0.2716],
   [0.2269, 0.2693, 0.6447, 0.6078, 0.6148]]])
  </code></pre>
  <hr />
  <p>I have tried gather(), but it does not work.</p>