input: Write a function to translate a sentence to Khmer using googletrans library.
  assert translate_sentence("Hello world") == "សួស្តី​ពិភពលោក"
  assert translate_sentence("Good morning") == "អរុណ​សួស្តី"
  assert translate_sentence("How are you?") == "តើ​អ្នក​សុខ​សប្បាយ​ទេ?"
output: def translate_sentence(sentence):
      from googletrans import Translator
      translator = Translator()
      translation = translator.translate(sentence, dest='km')
      return translation.text