fixing bug with multi-threaded access for variable length string arrays for onnx models. (#6537)

Author: michaelgsharp

src/Microsoft.ML.OnnxTransformer/OnnxTransform.cs

@@ -805,7 +805,7 @@ private class NamedOnnxValueGetterVec<T> : INamedOnnxValueGetter
                 public NamedOnnxValueGetterVec(DataViewRow input, int colIndex, OnnxShape tensorShape)
                 {
                     _srcGetter = input.GetGetter<VBuffer<T>>(input.Schema[colIndex]);
-                    _tensorShape = tensorShape;
+                    _tensorShape = new OnnxShape(tensorShape);
                     _colName = input.Schema[colIndex].Name;
                     _vBuffer = default;
                     _vBufferDense = default;