[tmva][sofie] Use -1 as default for batch_size in RFunction_Update::GenerateModel

The GenerateModel function called for the GNN classes was having a default =1 for the batch size used when generating the update models.
We should use -1 to not force any specific batch size and use directly the given input shape

Author: lmoneta

tmva/sofie/inc/TMVA/RFunction.hxx

@@ -52,8 +52,7 @@ public:
     std::shared_ptr<RModel> GetFunctionBlock() {
         return function_block;
     }
-
-    std::string GenerateModel(const std::string& filename, long read_pos=0, long block_size=1);
+    std::string GenerateModel(const std::string& filename, long read_pos = 0, long block_size = -1);
     std::string Generate(const std::vector<std::string>& inputPtrs);
     FunctionTarget GetFunctionTarget() {
         return fTarget;