title: RuntimeError: Only tuples, lists and Variables supported as JIT inputs, but got NoneType
body: <p>My code is </p>
  
  <pre><code>a=torch.randn(1,80,100,requires_grad=True)
  torch.onnx.export(waveglow,a, "waveglow.onnx")
  </code></pre>
  
  <p>I am trying to export a PyTorch model to ONNX format so i can use it in TensorRT. while testing my model in PyTorch the input tensor dimension is (1,80,x) where x varies depending on the input text length(the model i am using is TTS model named waveglow)</p>
  
  <p>when i try to run the above code for exporting the model to onnx, I am always getting this error</p>
  
  <pre><code>RuntimeError: Only tuples, lists and Variables supported as JIT inputs, but got NoneType
  </code></pre>
  
  <p>Please help</p>