If you go to Github of Fast.AI and search, you will find it in lots of places - Try this - https://github.com/fastai/fastai/search?q=F.sigmoid&unscoped_q=F.sigmoid 56
Its because Fast.AI Library currently is not (yet) updated to PyTorch 0.4.  They plan to do that with the next version of the course starting in middle of Oct so that the existing notebooks work and are aligned with their MOOC videos.  If you downgrade your PyTorch to 0.3.1, it should not give you these warnings.
Also, I see that you are not using an Virtual Env or Conda Env.  I would recommend that you create a FastAI Environment using steps in Readme at  https://github.com/fastai/fastai 15 and have a Separate Env for PyTorch 0.4 experiments.