torch.isnan
torch.isnan(input) -> Tensor
Returns a new tensor with boolean elements representing if each
   element of "input" is NaN or not. Complex values are considered NaN
   when either their real and/or imaginary part is NaN.
Parameters:
      input (Tensor) -- the input tensor.
Returns:
      A boolean tensor that is True where "input" is NaN and False
      elsewhere
Example:
  >>> torch.isnan(torch.tensor([1, float('nan'), 2]))
  tensor([False, True, False])
