docs: public DifferentialFunction getVariableOutputFunction(String variableName) {
          Preconditions.checkState(variables.containsKey(variableName), "No variable with name \"%s\" found in graph", variableName);
          if(variables.get(variableName).getOutputOfOp() == null)
              return null;
          return ops.get(variables.get(variableName).getOutputOfOp()).getOp();
      }