DOC: fix two build warnings (#41334)
Summary:
xref gh-38011.
Fixes two warnings when building documentation by
- using the external link to torchvision
- install tensorboard before building documentation
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41334
Reviewed By: ngimel
Differential Revision: D22753083
Pulled By: mruberry
fbshipit-source-id: 876377e9bd09750437fbfab0378664b85701f827
