code: def _covar_func(x1, x2, lengthscale, **kwargs):
      x1_, x2_ = _lazify_and_expand_inputs(x1, x2)
      lengthscale = lengthscale[..., None, None, 0, :]  # 1 x 1 x ndim
      # do not use .power(2.0) as it gives NaN values on cuda
      # seems related to https://github.com/getkeops/keops/issues/112
      K = (
          ((((x1_ - x2_).abs().sin()) ** 2) * (-2.0 / lengthscale))
          .sum(-1)
          .exp()
      )
      return K
  
file_path: gpytorch/gpytorch/kernels/keops/periodic_kernel.py
imports: import math
  from linear_operator.operators import KernelLinearOperator
  from ..periodic_kernel import PeriodicKernel
  from .keops_kernel import _lazify_and_expand_inputs, KeOpsKernel
label_desc: Use image node_id 1 for calling a global function with example usage: _covar_func(x1, x2, lengthscale) and returns: K