Remove bokeh dependency

[Innixma]

Issue #, if available:
#1051

Description of changes:

Removes bokeh dependency in autogluon.extra, which was only used optionally for plots. Since it caused numerous issues related to tornado dependencies in Kaggle (crash on importing AutoGluon), better to have it be purely optional, as a warning is shown to users who try to make plots telling them to install it.

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[szha]

Job PR-1055-1 is done.
Docs are uploaded to http://autogluon-staging.s3-website-us-west-2.amazonaws.com/PR-1055/1/index.html

[jwmueller]

Should there be some easy way for users to simultaneously install all optional dependencies like bokeh and others?
It can be inconvenient to have to first encounter the error message instructing how to install each one of these optional dependencies. Might be nicer to have a full install (including all optional dependencies) available via something like this:

pip install autogluon  # installs everything except autogluon.extra
pip install autogluon.extra

If we want the ENAS code to be non-optional, then it could be in autogluon.nas subpackage, which anyway is a clearer name than .extra.

@gradientsky your thoughts?

[Innixma]

Will merge for now, and we can refactor how we want to deal with generic optional dependencies in future