I guess I figured it out. This error seems to happen when I try to multiply those quantized tensors(input_x, mask). The workaround I took is:
# First, dequantize the quantized tensor
 input_x = self.dequant(input_x)
 mask = self.dequant(mask)
# Do the operation and quantize it back
 masked = input_x * mask
 masked = self.quant(masked)

 input_x = self.quant(input_x)
 mask = self.quant(mask)

 output = self.input_conv(masked)

Seems like pretty tedious work but it works. However, can I use self.quant() multiple times like that? or Should I use self.quant1(), self.quant2(), self.quant3() separately?