class_docstr: Differentiable relaxation of the L0 norm using a Gaussian basis function.
code: def __call__(self, X: Tensor) -> Tensor:
      return nnz_approx(X=X, target_point=self.target_point, a=self.a)
  
imports: from __future__ import annotations
  import math
  from typing import Any, Callable, List, Optional
  import torch
  from botorch.acquisition.acquisition import AcquisitionFunction
  from botorch.acquisition.analytic import AnalyticAcquisitionFunction
  from botorch.acquisition.objective import GenericMCObjective
  from botorch.exceptions import UnsupportedError
  from torch import Tensor
label_desc: Use image node_id 2 for calling the L0Approximation obj's underlying member method code with example usage: obj.__call__(X) and returns: nnz_approx