[ROOT-10909] Add TMVA python dependencies to the requirements.txt

[vepadulano]

Explain what you would like to see improved and how.

From https://its.cern.ch/jira/browse/ROOT-10909

The PR #5408 will add a requirements.txt file to the repo, which should reflect our dependencies to python packages. The current status does not cover TMVA with xgboost, sklearn and keras as runtime dependencies, which should be added.

ROOT version

Any

Installation method

Any

Operating system

Any

Additional context

No response

[vepadulano]

Our current understanding is that we lack the following Python packages for the optional runtime dependencies of TMVA

tensorflow 
keras
torch
sklearn
xgboost
sonnet
graph_nets

[dpiparo]

@lmoneta could you confirm this is the list of dependencies? It's important to have those fixed, also considering containers and setup of CI nodes.

[lmoneta]

Yes, this is the list of dependencies, but they are optional, we should not emit an error if the package is not installed

[dpiparo]

but what happens if they are not there?

[lmoneta]

For TMVA nothing, if they are not there the corresponding tests are not run. Everything is protected. But I think if we add in the requirements.txt and the package is not there an error in the build is emitted.
We should have a way to install the packages in the new CI, but not in all platforms, only in some of them, for example in a platform tensorflow in another torch. It could be too much work to install them in every platforms, due to some possible incompatibilities

[dpiparo]

@lmoneta I was wondering if this PR could be merged after all