input: Write a python function to split a sentence into a list of words.
  assert split_sentence("The quick brown fox jumps over the lazy dog") == ['The', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'lazy', 'dog']
  assert split_sentence("Hello world") == ['Hello', 'world']
  assert split_sentence("A quick test") == ['A', 'quick', 'test']
output: def split_sentence(sentence):
      return sentence.split()