build fails in CI - blocking PRs

[aaronmarkham]

The error/warning is not very helpful.

http://ci.mxnet.io/blue/organizations/jenkins/mxnet-new-docs/detail/aaron_porting_content/7/pipeline

The build seems fine when I run with #70 and completes on my staging server.

[aaronmarkham]

Looks like CI is using Sphinx 2.0.1 now, and on a previous build of the branch's run it used 1.8.5.
@astonzhang - we should try to resolve the error on 2.0.1 and go with the newer version.

[aaronmarkham]

Tested my setup by upgrading the environment and it works fine with v2.0.1. Not sure why CI is breaking.

[szha]

I recall we had weird issues in gluonnlp because of sphinx 2.0 too. in our case it was likely related to the compatibility issue with nbsphinx. Here are the versions of packages that we froze to for building doc: https://github.com/dmlc/gluon-nlp/blob/master/env/doc.yml

[aaronmarkham]

I'm able to build locally and the CI seems to pass intermittently. It failed again though.
http://ci.mxnet.io/job/mxnet-new-docs/job/installation_updates/1/display/redirect