input: Write a python function to count the number of characters in each word in a given sentence and return a list of these counts.
  assert count_characters("Hello World!") == [5, 6]
  assert count_characters("Python is fun") == [6, 2, 3]
  assert count_characters("OpenAI creates AI models") == [6, 7, 2, 6]
output: def count_characters(sentence):
      return [len(word) for word in sentence.split()]