input: Write a python function to add a word to the nested dictionary.
  assert add_word(words_dict, "baz") == {"hello": {}, "world": {}, "foo": {}, "bar": {}, "baz": {}}
  assert add_word(words_dict, "python") == {"hello": {}, "world": {}, "foo": {}, "bar": {}, "baz": {}, "python": {}}
  assert add_word(words_dict, "example") == {"hello": {}, "world": {}, "foo": {}, "bar": {}, "baz": {}, "python": {}, "example": {}}
output: def add_word(words_dict, word):
      words_dict[word] = {}
      return words_dict