Solved by jerryzh168 in post #2 
              
              
                This means the input of aten::add_ is a quantized Tensor. To address the problem you can either 
(1). place a DequantStub and QuantStub around the aten::add_ op. 
e.g. 
def __init__(...):
    ...
    self.quant = torch.quantization.QuantStub()
    self.dequant = torch.quantization.DeQuantStub()
   …