Fix TrackioCallback fails to log evaluation metrics after training ends

[lewtun]

What does this PR do?

This PR fixes the following issue: TrackioCallback.on_train_end() calls trackio.finish(), which clears Trackio’s active run. However, the callback keeps _initialized=True. If user code calls trainer.evaluate() or trainer.predict() after trainer.train(), the callback later receives on_log()/on_predict(). Because _initialized is still True, it skips setup() and calls trackio.log() without an active Trackio run, raising:

RuntimeError: Call trackio.init() before trackio.log().

The fix is to reset TrackioCallback._initialized after trackio.finish() in on_train_end(). Then subsequent logging re-runs setup() and reinitializes/resumes the Trackio run before logging metrics.

I've also added a regression test which triggers the problem on main and verified the fix resolves it.

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[lewtun]

I added a regression test since we didn't have any at all for trackio. Happy to remove it if that's preferred

[lewtun]

The failing tests seem unrelated to my changes

[github-actions]

CI Dashboard: View test results in Grafana

[qgallouedec]

Didn't try myself but the justification and fix lgtm