Add katex to pytorch-linux-xenial-py3.6-gcc5.4 docker image (#30522)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30522
This is in preparation for moving the docs push CI jobs to depend on
`pytorch-linux-xenial-py3.6-gcc5.4` rather than
`pytorch-linux-xenial-cuda9-cudnn7-py3`.
Test Plan: Imported from OSS
Differential Revision: D18731108
Pulled By: zou3519
fbshipit-source-id: fd753a5ca818fa73a14e4276c33368a247cc40e1
