Hard code to mannually make sigma to 0

Hi

I saw a piece of code in generator.py to set the sigma to 0. I am very confused as to why you set it like this. What benefits would it bring?

``` python
                padd = 0.1
                mask_box = torch.all(
                    p_i <= 1. + padd, dim=-1) & torch.all(
                        p_i >= -1. - padd, dim=-1)
                sigma_i[mask_box == 0] = 0.
```