fix: add comprehensive validation to MultiModalTeacherModel constructor

The constructor was dividing by modalityTeachers.Length (line 24) and accessing
_modalityTeachers[0] (lines 14, 36) without validating the array, which could
cause DivideByZeroException, IndexOutOfRangeException, or inconsistent behavior.

Added validation to fail fast in the constructor:
1. Check that modality teachers array is non-empty (Length > 0)
2. Check that no teacher is null
3. Check that all teachers have the same OutputDimension
4. Improved modalityWeights validation with clearer error message

This prevents runtime errors when OutputDimension or GetLogits is invoked
and provides clear, actionable error messages indicating which validation failed.

Similar to the fix applied to DistributedTeacherModel.

Author: claude

src/KnowledgeDistillation/Teachers/MultiModalTeacherModel.cs

@@ -19,14 +19,40 @@ public MultiModalTeacherModel(
     {
         _modalityTeachers = modalityTeachers ?? throw new ArgumentNullException(nameof(modalityTeachers));
 
+        // Validate modality teachers array is non-empty
+        if (_modalityTeachers.Length == 0)
+            throw new ArgumentException("Modality teachers array cannot be empty", nameof(modalityTeachers));
+
+        // Validate no teacher is null
+        for (int i = 0; i < _modalityTeachers.Length; i++)
+        {
+            if (_modalityTeachers[i] == null)
+                throw new ArgumentException($"Modality teacher at index {i} is null", nameof(modalityTeachers));
+        }
+
+        // Validate all teachers have the same output dimension
+        int expectedOutputDim = _modalityTeachers[0].OutputDimension;
+        for (int i = 1; i < _modalityTeachers.Length; i++)
+        {
+            if (_modalityTeachers[i].OutputDimension != expectedOutputDim)
+                throw new ArgumentException(
+                    $"Modality teacher at index {i} has OutputDimension {_modalityTeachers[i].OutputDimension}, " +
+                    $"but expected {expectedOutputDim} (from teacher 0). " +
+                    $"All modality teachers must have the same OutputDimension.",
+                    nameof(modalityTeachers));
+        }
+
+        // Set or validate modality weights
         if (modalityWeights == null)
         {
             _modalityWeights = Enumerable.Repeat(1.0 / modalityTeachers.Length, modalityTeachers.Length).ToArray();
         }
         else
         {
             if (modalityWeights.Length != modalityTeachers.Length)
-                throw new ArgumentException("Modality weights must match number of teachers");
+                throw new ArgumentException(
+                    $"Modality weights length ({modalityWeights.Length}) must match number of teachers ({modalityTeachers.Length})",
+                    nameof(modalityWeights));
             _modalityWeights = modalityWeights;
         }
     }