[Data] Only show message regarding hiding op-level progress bar once

[scottjlee]

Why are these changes needed?

#46826 introduced a bug, where the info log regarding hiding operator-level progress bars is always shown, regardless of whether the code was run via a Ray Job or not.

This PR fixes the bug by moving the check for whether the code is run via a Ray Job or not into the DataContext.__post_init__() method, so that the check is done only after the DataContext singleton is initialized.

Related issue number

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  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
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• Testing Strategy
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  • This PR is not tested :(

[omatthew98]

one step forwards one step backwards, but we march the progress bar onwards.