class_docstr: Ishigami test function.
  
  three-dimensional function (usually evaluated on `[-pi, pi]^3`):
  
      f(x) = sin(x_1) + a sin(x_2)^2 + b x_3^4 sin(x_1)
  
  Here `a` and `b` are constants where a=7 and b=0.1 or b=0.05
  Proposed to test sensitivity analysis methods because it exhibits strong
  nonlinearity and nonmonotonicity and a peculiar dependence on x_3.
code: def _optimal_value(self) -> float:
      raise NotImplementedError
  
imports: import math
  from typing import List, Optional, Tuple
  import torch
  from botorch.test_functions.synthetic import SyntheticTestFunction
  from torch import Tensor
label_desc: Use image node_id 2 for calling the Ishigami obj's underlying member method code with example usage: obj._optimal_value() without return types