input: Write a python function to print all paths in an m x n grid.
  assert print_all_paths(2, 2) == None
  assert print_all_paths(3, 2) == None
  assert print_all_paths(2, 3) == None
output: def print_all_paths(m, n):
      def backtrack(path, x, y):
          if x == m - 1 and y == n - 1:
              print(path)
              return
          if x < m - 1:
              backtrack(path + 'D', x + 1, y)
          if y < n - 1:
              backtrack(path + 'R', x, y + 1)
  
      backtrack('', 0, 0)