I see.
To summary what I understood, the quantization step is done as follow.

Load pretrained fp32 model
run prepare() to prepare converting pretrained fp32 model to int8 model
run fp32model.forward() to calibrate fp32 model by operating the fp32 model for a sufficient number of times. However, this calibration phase is a kind of `blackbox’ process so I cannot notice that the calibration is actually done.
run convert() to finally convert the calibrated model to usable int8 model.