fix: re-add invalidate_cache! for use with new metadata

Author: AayushSabharwal

src/structural_transformation/StructuralTransformations.jl

@@ -21,7 +21,7 @@ using ModelingToolkit: System, AbstractSystem, var_from_nested_derivative, Diffe
                        has_tearing_state, defaults, InvalidSystemException,
                        ExtraEquationsSystemException,
                        ExtraVariablesSystemException,
-                       vars!,
+                       vars!, invalidate_cache!,
                        IncrementalCycleTracker, add_edge_checked!, topological_sort,
                        filter_kwargs, lower_varname_with_unit,
                        lower_shift_varname_with_unit, setio, SparseMatrixCLIL,

src/systems/abstractsystem.jl

@@ -899,6 +899,13 @@ end
 
 has_equations(::AbstractSystem) = true
 
+"""
+    $(TYPEDSIGNATURES)
+
+Invalidate cached jacobians, etc.
+"""
+invalidate_cache!(sys::AbstractSystem) = sys
+
 function Setfield.get(obj::AbstractSystem, ::Setfield.PropertyLens{field}) where {field}
     getfield(obj, field)
 end