input: Write a python function to multiply two 2D numpy arrays using np.matmul.
  assert np.array_equal(matmul([[1, 2, 3], [4, 5, 6]], [[1, 2], [3, 4], [5, 6]]), np.array([[22, 28], [49, 64]]))
  assert np.array_equal(matmul([[1, 0], [0, 1]], [[4, 1], [2, 2]]), np.array([[4, 1], [2, 2]]))
  assert np.array_equal(matmul([[2, 4], [1, 3]], [[3, 1], [2, 5]]), np.array([[14, 22], [ 9, 16]]))
output: import numpy as np
  
  def matmul(a, b):
      return np.matmul(a, b)