TEST: State requirements for conditional tests

[david-cortes-intel]

Description

This PR adds a file which explicitly lists optional requirements which are used for tests, but which the tests conditionally check for whether the dependency is installed or not (e.g. requirements that are only used for GPU or distributed mode).

This file is currently not used for anything, since all CI jobs that test extra functionality (GPU, distributed, etc.) already install these through other means, and the versions are not pinned since different CI jobs also use different versions and there are no clear requirements around them.

Would be nice to hear opinions about potential other ways to use this file.

Note: I'm not 100% sure that this covers all of the optional dependencies for non-base tests.

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Checklist to comply with before moving PR from draft:

PR completeness and readability

• I have reviewed my changes thoroughly before submitting this pull request.
• Git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details).
• I have added a respective label(s) to PR if I have a permission for that.
• I have resolved any merge conflicts that might occur with the base branch.

Testing

Not applicable.

Performance

Not applicable.

[icfaust]

I'm not opposed to this PR, its definitely some stuff that lacks clarity in the codebase currently. I think its worth having an internal discussion how to iron out all of this into a consistent way for external developers/users.

[ethanglaser]

I'm not opposed to this PR, its definitely some stuff that lacks clarity in the codebase currently. I think its worth having an internal discussion how to iron out all of this into a consistent way for external developers/users.

Agreed - I think this is something that should be added but not in an additional requirements file. We have started to make some progress towards unifying location of deps but more work to be done here in Q4