annotate FunctionMixin methods _ad_mul and _ad_add (#3841)

* annotate FunctionMixin _ad_mul and _ad_add

* FunctionMixin _ad_mul and _ad_add use Function operations not Vector operations

firedrake/adjoint_utils/function.py

@@ -300,7 +300,6 @@ def _ad_will_add_as_dependency(self):
         with checkpoint_init_data():
             super()._ad_will_add_as_dependency()
 
-    @no_annotations
     def _ad_mul(self, other):
         from firedrake import Function
 
@@ -309,7 +308,6 @@ def _ad_mul(self, other):
         r.assign(other * self)
         return r
 
-    @no_annotations
     def _ad_add(self, other):
         from firedrake import Function
 
@@ -363,16 +361,12 @@ def _ad_dim(self):
         return self.function_space().dim()
 
     def _ad_imul(self, other):
-        vec = self.vector()
-        vec *= other
+        self *= other
+        return self
 
     def _ad_iadd(self, other):
-        vec = self.vector()
-        ovec = other.vector()
-        if ovec.dat == vec.dat:
-            vec *= 2
-        else:
-            vec += ovec
+        self += other
+        return self
 
     def _ad_function_space(self, mesh):
         return self.ufl_function_space()