input: Write a function to generate a 9x9 Sudoku puzzle board.
  assert len(generate_sudoku()) == 9
  assert len(generate_sudoku()[0]) == 9
  assert all(1 <= cell <= 9 for row in generate_sudoku() for cell in row)
  assert all(len(set(row)) == 9 for row in generate_sudoku())
output: def generate_sudoku():
      import random
  
      def is_valid(board, row, col, num):
          for i in range(9):
              if board[row][i] == num or board[i][col] == num:
                  return False
          start_row, start_col = 3 * (row // 3), 3 * (col // 3)
          for i in range(3):
              for j in range(3):
                  if board[start_row + i][start_col + j] == num:
                      return False
          return True
  
      def solve_sudoku(board):
          for row in range(9):
              for col in range(9):
                  if board[row][col] == 0:
                      for num in range(1, 10):
                          if is_valid(board, row, col, num):
                              board[row][col] = num
                              if solve_sudoku(board):
                                  return True
                              board[row][col] = 0
                      return False
          return True
  
      def remove_numbers(board, num_holes):
          holes_set = set()
          while len(holes_set) < num_holes:
              i, j = random.randint(0, 8), random.randint(0, 8)
              if (i, j) not in holes_set:
                  holes_set.add((i, j))
                  board[i][j] = 0
  
      board = [[0 for _ in range(9)]]
      solve_sudoku(board)
      remove_numbers(board, random.randint(20, 40))
      return board