Consider a Hamiltonian H=x^2 + y^2. I want automatically calculate Hamilton’s equation of motion by taking the partial derivatives wrt to x and y. How to do this with ModelingToolkit.jl? I cannot get `calculate_gradient`

to work:

```
@ variables x y
H = x^2 + y^2
calculate_gradient(H)
```

results in

```
MethodError: no method matching calculate_gradient(::Symbolics.Num)
Closest candidates are:
calculate_gradient(!Matched::ModelingToolkit.OptimizationSystem)
@ ModelingToolkit C:\Users\Goran\.julia\packages\ModelingToolkit\xLMQM\src\systems\optimization\optimizationsystem.jl:118
```