title: Float16 (HalfTensor) in pytorch + cuda
body: <p>Can I set <code>torch.HalfTensor</code> as default and use it with CUDA?</p>
  
  <p>I can't even create usual Conv2D:</p>
  
  <pre><code>In [1]: import torch 
  In [2]: torch.__version__
  Out[2]: '0.2.0_3'
  
  In [3]: from torch import nn
  
  In [4]: torch.set_default_tensor_type('torch.HalfTensor')
  
  In [5]: conv1 = nn.Conv2d(1, 64, kernel_size=3, stride=1, padding=1, bias=False)
  terminate called after throwing an instance of 'std::invalid_argument'
    what():  Unsupported tensor type
  Aborted (core dumped)
  </code></pre>
  
  <p><strong>UPD:</strong> it doesn't work without CUDA too but I'm interested in case <em>with</em> CUDA support.</p>