Boring default setting: will uninstall the installed torch which makes trouble.

When run the `pip install torchdyn`, it automatically installs the torch  and uninstalls the old version installed in the environment.

This makes touble if we have already installed a good version (especially the cuda version) of PyTorch, and should install the cuda torch again.

It is suggested to make a note telling people to watch out the torch version to avoid such issues.