Update contributing doc after walkthrough

[tengyifei]

Since the docker container runs as root, if we chown -R root:root ., that will cause vscode to be unable to load any files the workspace due to permission denied. That's probably not reasonable for local development. Instead I changed to trust repositories from another user.

Added torchvision install instructions since some tests imports torchvision.

The libtpu install instructions tell us to install torch_xla[tpu]. IIUC that will just overwrite whatever code we just installed locally with an official build, which is probably not what we want. I changed that to use the "extras" feature from setup.py.