[timeseries] Fix PerformanceWarning in AutoGluonTabularModel

[shchur]

Description of changes:

• When training AutoGluonTabular model with both past & known covariates, the following warning is raised by pandas

/local/home/shchuro/workspace/autogluon/timeseries/src/autogluon/timeseries/models/autogluon_tabular/tabular_model.py:230: PerformanceWarning: DataFrame is highly fragmented. This is usually the result of calling frame.insert many times, which has poor performance. Consider joining all columns at once using pd.concat(axis=1) instead. To get a de-fragmented frame, use newframe = frame.copy()

This happens at line

  features[self.target] = time_series[self.target]

This PR replaces column insertion with a concatenation, which fixes this warning.

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[github-actions]

Job PR-2817-e85f7ae is done.
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