add model cloning to model_builder python + sample

ortools/linear_solver/python/model_builder.py

@@ -631,6 +631,12 @@ class ModelBuilder:
     def __init__(self):
         self.__helper: mbh.ModelBuilderHelper = mbh.ModelBuilderHelper()
 
+    def clone(self) -> "ModelBuilder":
+        """Returns a clone of the current model."""
+        clone = ModelBuilder()
+        clone.helper.overwrite_model(self.helper)
+        return clone
+
     @typing.overload
     def _get_linear_constraints(self, constraints: Optional[pd.Index]) -> pd.Index:
         ...

ortools/linear_solver/python/model_builder_helper.cc

@@ -169,6 +169,7 @@ PYBIND11_MODULE(model_builder_helper, m) {
 
   py::class_<ModelBuilderHelper>(m, "ModelBuilderHelper")
       .def(py::init<>())
+      .def("overwrite_model", &ModelBuilderHelper::OverwriteModel, arg("other_helper"))
       .def("export_to_mps_string", &ModelBuilderHelper::ExportToMpsString,
            arg("options") = MPModelExportOptions())
       .def("export_to_lp_string", &ModelBuilderHelper::ExportToLpString,