code: def _linear_kernel(X, Y):
      return np.dot(X, Y.T)
  
imports: import re
  import numpy
  import pytest
  from sklearn.datasets import make_classification
  from sklearn.kernel_approximation import AdditiveChi2Sampler, Nystroem, PolynomialCountSketch, RBFSampler, SkewedChi2Sampler
  from sklearn.metrics.pairwise import chi2_kernel, kernel_metrics, polynomial_kernel, rbf_kernel
  from sklearn.utils._testing import assert_allclose, assert_array_almost_equal, assert_array_equal
  from sklearn.utils.fixes import CSR_CONTAINERS
label_desc: Use image node_id 3 for calling a global function with example usage: _linear_kernel(X, Y) and returns: np