Fix CodeQL warning: __init__ calls overridden method prepare_pipeline

Problem

CodeQL alert #2 flags py/init-calls-subclass in base_evaluator.py:46.

WinMLEvaluator.__init__ calls self.prepare_pipeline(), which is overridden by 4 subclasses:

• WinMLImageSegmentationEvaluator
• WinMLObjectDetectionEvaluator
• WinMLTextClassificationEvaluator
• WinMLTokenClassificationEvaluator

This means subclass overrides receive a partially initialized instance. Currently safe because subclasses set their own attributes before super().__init__(), but fragile for future changes.

Proposed Fix

Convert data and pipe to lazy properties in the base class. prepare_pipeline / prepare_data are called on first access instead of during __init__, so the object is fully constructed by then.

• Change is confined to base_evaluator.py
• Zero subclass modifications needed
• Transparent to callers (self.pipe / self.data usage unchanged)