input: Write a python function to convert a query string into a dictionary.
  assert query_to_dict("student=John&grade=A") == {'student': 'John', 'grade': 'A'}
  assert query_to_dict("name=Alice&age=30&country=USA") == {'name': 'Alice', 'age': '30', 'country': 'USA'}
  assert query_to_dict("key1=value1&key2=value2&key3=value3") == {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}
output: def query_to_dict(query_string):
      return dict(item.split("=") for item in query_string.split("&"))