Check out here (Quantization — PyTorch 1.9.1 documentation) for a description of this error, it means you are passing a quantized tensor to a fp32 kernel.
For eager mode quantization, you could edit the model code to replace the addition with FloatFunctional.
For a backbone such as ResNet18, you could also use FX graph mode quantization which will do the syntax transforms for you automatically.  You could check out here (Quantization — PyTorch 1.9.1 documentation 3) for an API example (scroll down to FX Graph Mode Quantization).