input: Write a python function to capitalize each word in a sentence.
  assert capitalize_sentence('i am learning python') == 'I Am Learning Python'
  assert capitalize_sentence('hello world') == 'Hello World'
  assert capitalize_sentence('this is a test sentence') == 'This Is A Test Sentence'
output: def capitalize_sentence(sentence):
      return ' '.join(word.capitalize() for word in sentence.split())