Replace _SpatialGaussianKernel with torchvision.transforms.GaussianBlur

[sandylaker]

https://github.com/YaNgZhAnG-V5/informationbottleneck/blob/775fad551e900907010ae8914fe727cb8e126d82/iba/models/utils.py#L48

torchvision.transforms.GaussianBlur also supports Tensor input.