title: Selecting second dim of tensor using an index tensor
body: <p>I have a 2D tensor and an index tensor.  The 2D tensor has a batch dimension, and a dimension with 3 values.  I have an index tensor that selects exactly 1 element of the 3 values.  What is the "best" way to product a slice containing just the elements in the index tensor?</p>
  
  <pre><code>t = torch.tensor([[1,2,3], [4,5,6], [7,8,9]])
  t = tensor([[1, 2, 3],
          [4, 5, 6],
          [7, 8, 9]])
  
  i = torch.tensor([0,0,1], dtype=torch.int64)
  tensor([0, 0, 1])
  </code></pre>
  
  <p>Expected output...</p>
  
  <pre><code>tensor([1, 4, 8])
  </code></pre>