DOC: fix two build warnings (pytorch#41334)

Summary:
xref pytorchgh-38011.

Fixes two warnings when building documentation by
- using the external link to torchvision
- install tensorboard before building documentation

Pull Request resolved: pytorch#41334

Reviewed By: ngimel

Differential Revision: D22753083

Pulled By: mruberry

fbshipit-source-id: 876377e9bd09750437fbfab0378664b85701f827

Author: mattip

docs/requirements.txt

@@ -2,3 +2,4 @@ sphinx==2.4.4
 -e git+https://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme
 sphinxcontrib.katex
 matplotlib
+tensorboard

docs/source/index.rst

@@ -70,7 +70,7 @@ PyTorch is an optimized tensor library for deep learning using GPUs and CPUs.
 
    torchaudio <https://pytorch.org/audio>
    torchtext <https://pytorch.org/text>
-   torchvision/index
+   torchvision <https://pytorch.org/vision>
    TorchElastic <https://pytorch.org/elastic/>
    TorchServe <https://pytorch.org/serve>
    PyTorch on XLA Devices <http://pytorch.org/xla/>