Model according to freezeing regression head

[dassaniansh]

@bw4sz @henrykironde, I have opened this pull request according to the way you people have suggested. I have added the config argument 'freeze_regression_head' and I have also loaded the trained model in the init function. Now as per the steps you have suggested I believe I have to create a new model with classes given by the user which accordingly can distinguish between categories but I'm not completely sure how to move forward with that, a little bit of guidance would be really helpfull.