title: If I'm not specifying to use CPU/GPU, which one is my script using?
body: <p>In pytorch, if I'm not writing anything about using CPU/GPU, and my machine supports CUDA (<code>torch.cuda.is_available() == True</code>):</p>
  
  <ol>
  <li>What is my script using, CPU or GPU?</li>
  <li>If CPU, what should I do to make it run on GPU? Do I need to rewrite everything?</li>
  <li>If GPU, will this script crash if <code>torch.cuda.is_available() == False</code>?</li>
  <li>Does this do anything about making the training faster?</li>
  <li>I'm aware of <a href="https://stackoverflow.com/questions/46704352/porting-pytorch-code-from-cpu-to-gpu?rq=1">Porting PyTorch code from CPU to GPU</a> but this is old. Does this situation change in v0.4 or the upcoming v1.0?</li>
  </ol>