[BE] differentiate the inner exp of getPart

Belonging to [master]:
  - #2276

Author: Willi Braun

Compiler/BackEnd/Differentiate.mo

@@ -1462,7 +1462,10 @@ algorithm
       exp_1 = Expression.makeZeroExpression(Expression.arrayDimension(tp));
     then (exp_1, inFuncs);
 
-    case ("$getPart",_) then (exp, inFuncs);
+    case ("$getPart",_) equation
+        (exp_1, funcs) = differentiateExp(exp, inDiffwrtCref, inInputData,inDiffType,inFuncs, maxIter);
+    then (exp_1, funcs);
+
     case ("firstTick",_) then (exp, inFuncs);
     case ("interval",_) then (exp, inFuncs);