input: Write a python function to reverse each row of a given matrix.
  assert reverse_row([[1, 2, 3], [4, 5, 6]]) == [[3, 2, 1], [6, 5, 4]]
  assert reverse_row([[7, 8, 9], [10, 11, 12]]) == [[9, 8, 7], [12, 11, 10]]
  assert reverse_row([[1]]) == [[1]]
output: def reverse_row(matrix):
      return [row[::-1] for row in matrix]