Switch eval yaml configs to use evaluation_platform

[wizeng23]

Description

evaluation_platform was renamed to evaluation_backend throughout our codebase in #1484. This causes an error in all our eval configs at main, which use the YAML config from HEAD but the Oumi package from PyPi (which hasn't picked up this rename).

This PR:

• Renames evaluation_backend back to evaluation_platform to fix the breakage
• Updates EvaluationTaskParams so that evaluation_backend is not a required field. This will cause a breakage on main, as our configs don't provide this value anymore. Also updates post_init logic in this class to expect one of the two fields to be populated, not none or both.
• Fixes a test

After the next PyPI push, we can revert this change.

Tested that there's no errors both when using the PyPI package and using source.

Related issues

Towards OPE-1097

Before submitting

• This PR only changes documentation. (You can ignore the following checks in that case)
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• Did you add / update tests where needed?