Make tensorflow optional

[maccam912]

I am operating in a very low disk space environment, and Tensorflow as a dependency is giving me an extra 500 MB of download I don't need. I have moved it here to an optional extra, so a 'pip install causalml[tensorflow]' will still get it, but an installation of causalml without specifying tensorflow will only require xgboost.

All existing tests run when following CONTRIBUTING.md guidelines.

@ppstacy @paullo0106 @waltherg @jeongyoonlee @ccrndn @zhenyuz0500 and please tag anyone else I missed.

[maccam912]

Related to issue #253

[jeongyoonlee]

Thanks for the PR, @maccam912! Although, it makes sense to exclude tensorflow in your use case, I am not sure about removing it from the default pip installation as this PR will do.

Instead, what about using --no-deps in your use case?

It'd be ideal if there's a way to exclude a certain dependency optionally ,but I couldn't find it.

Let me know what you think.

Thanks!

[maccam912]

You spend much more time with causalml than I do, so I defer to your judgement. I can close my PR, and try the --no-deps or use my fork if I need to use the reduced size download.

[jeongyoonlee]

Thanks. I'll look forward to more contribution from you! :)