[NFC] Make Generator class becomes ABC

[jerryzj]

• The following member functions will return NotImplemented if not implemented in derived classes to let user aware that the function call is not functioning

  • write()
  • write_title()
  • gen_prologue()
  • inst_group_prologue()
  • inst_group_epilogue()
  • post_gen()

• The func function is set as abstract method, all the derived classes shoule have their own implementation

• The original func implementation is copied to DocGenerator and APITestGenerator and replace the call of base func() implementation