Fix colab notebook

[amrzv]

Hi. Made some changes to notebook:

• fixed RuntimeError #21, #32, that was fixed in #31 and #37, but not for msgnet.ipynb;
• removed unused import torch.nn.functional;
• prettified according to pep8;
• changed os.system('wget ...') to direct calling !wget ... without importing os module.

Tested in colab (run all), the notebook works as expected without errors.

[zhanghang1989]

Thanks a lot!