fix: remove default! null-forgiving operator from relationalDistillationStrategy

Removed the null-forgiving operator (default!) from _accumulatedRelationalLoss field
and replaced with proper initialization using NumOps.Zero in the constructor.
This ensures production-ready null handling without suppressing compiler warnings.

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ooples

src/KnowledgeDistillation/Strategies/RelationalDistillationStrategy.cs

@@ -82,7 +82,7 @@ public class RelationalDistillationStrategy<T> : DistillationStrategyBase<T, Vec
     // Batch accumulation buffers for relational loss computation
     private readonly List<Vector<T>> _batchStudentOutputs = new();
     private readonly List<Vector<T>> _batchTeacherOutputs = new();
-    private T _accumulatedRelationalLoss = default!;
+    private T _accumulatedRelationalLoss;
     private Vector<T>? _cachedRelationalGradient = null; // Cached average gradient for amortization
     private int _samplesSinceRelationalCompute = 0;
     private int _actualBatchCountForAmortization = 0; // Tracks actual count used for loss computation