Results 3 issues of Baimu

I directly download the pretrained model by `mask_encoder = torch.hub.load('milesial/Pytorch-UNet', 'unet_carvana', pretrained=True, scale=0.5)` but it seems have some error ``` File "/home/xxx/anaconda3/envs/pytorch/lib/python3.7/site-packages/torch/hub.py", line 404, in load model = _load_local(repo_or_dir, model,...

review all content again and fix the bug. Update the docs and the notebook in ch12