class_docstr: Gsobol test function.
  
  d-dimensional function (usually evaluated on `[0, 1]^d`):
  
      f(x) = Prod_{i=1}\^{d} ((\|4x_i-2\|+a_i)/(1+a_i)), a_i >=0
  
  common combinations of dimension and a vector:
  
      dim=8, a= [0, 1, 4.5, 9, 99, 99, 99, 99]
      dim=6, a=[0, 0.5, 3, 9, 99, 99]
      dim = 15, a= [1, 2, 5, 10, 20, 50, 100, 500, 1000, ..., 1000]
  
  Proposed to test sensitivity analysis methods
  First order Sobol indices have closed form expression S_i=V_i/V with :
  
      V_i= 1/(3(1+a_i)\^2)
      V= Prod_{i=1}\^{d} (1+V_i) - 1
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 Gsobol obj's underlying member method code with example usage: obj._optimal_value() without return types