docstr: Kernel for a single layer
code: def _single_layer_K(
      self, x1norm, x2norm, x1sum, x2sum, cos_theta, x2_none=False
  ):
      """
      Kernel for a single layer
      """
      if self.cos_or_sin == "half":
          return np.cos(x1sum - x2sum) * np.exp(
              -0.5
              * (
                  x1norm**2
                  + x2norm**2
                  - 2 * x1norm * x2norm * cos_theta
              )
          )
  
      term1 = np.cos(x1sum - x2sum) * np.exp(
          -0.5
          * (
              x1norm**2
              + x2norm**2
              - 2 * x1norm * x2norm * cos_theta
          )
      )
      term2 = np.cos(x1sum + x2sum) * np.exp(
          -0.5
          * (
              x1norm**2
              + x2norm**2
              + 2 * x1norm * x2norm * cos_theta
          )
      )
      if self.cos_or_sin == "cos":
          return term1 + term2
      elif self.cos_or_sin == "sin":
          return term1 - term2
  
file_path: elu-gelu-kernels/code/kernels/nn_kernel_trig.py
imports: import numpy
  from scipy.stats import multivariate_normal
  from scipy.stats import norm
  from scipy.special import erf
  import GPy
  import abc
  from .nn_kernel import NNKernel
label_desc: Use image node_id 2 for calling the NNKernelTrig obj's underlying member method code with example usage: obj._single_layer_K(x1norm, x2norm, x1sum, x2sum, cos_theta, x2_none) and returns: unknown, unknown, unknown