Missing using ImmutableObjectGraph when model has ImmutableList<T> fields

[israellot]

It seems the generated code uses a extension method ResetContents that requires
using ImmutableObjectGraph; which is not present on the generated file.
It happens when use fields of type ImmutableList

[israellot]

using ImmutableObjectGraph to original source files solve the compilation error, but is not required on original source files since the [GenerateImmutable] attribute lives in ImmutableObjectGraph.Generation and VS can remove the unused directive on cleanup.