input: Write a function to count the occurrences of each letter in a given sentence.
  assert count_letter_occurrences('The quick brown fox jumps over the lazy dog.')=={'T': 1, 'h': 2, 'e': 3, ' ': 8, 'q': 1, 'u': 2, 'i': 1, 'c': 1, 'k': 1, 'b': 1, 'r': 2, 'o': 4, 'w': 1, 'n': 1, 'f': 1, 'x': 1, 'j': 1, 'm': 1, 'p': 1, 's': 1, 'v': 1, 't': 1, 'l': 1, 'a': 1, 'z': 1, 'y': 1, 'd': 1, 'g': 1, '.': 1}
  assert count_letter_occurrences('hello world')=={'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, 'r': 1, 'd': 1}
  assert count_letter_occurrences('aa bb cc')=={'a': 2, ' ': 2, 'b': 2, 'c': 2}
output: def count_letter_occurrences(sentence):
      occurrences = {}
      for letter in sentence:
          if letter in occurrences:
              occurrences[letter] += 1
          else:
              occurrences[letter] = 1
      return occurrences