Fix relation between IsSavedModel and isFrozen (dotnet#4197)

IsSavedModel returns true when loaded model is an-frozen model (ref: https://github.com/dotnet/machinelearning/blob/1503b0aa9cac997cff8b8bc7e2075eb23d61ad81/src/Microsoft.ML.Dnn/DnnUtils.cs#L137 )
but now isFrozen variable is set true in spite of an-frozen model.

Author: yamachu

src/Microsoft.ML.Dnn/DnnRetrainTransform.cs

@@ -683,7 +683,7 @@ private protected override void SaveModel(ModelSaveContext ctx)
             // for each output column
             //   int: id of output column name
             // stream: tensorFlow model.
-            var isFrozen = DnnUtils.IsSavedModel(_env, _modelLocation);
+            var isFrozen = !DnnUtils.IsSavedModel(_env, _modelLocation);
             ctx.Writer.WriteBoolByte(isFrozen);
             ctx.Writer.WriteBoolByte(_addBatchDimensionInput);