input: Write a python function to count the occurrences of each letter in a string.
  assert letter_count("hello world") == {'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, 'r': 1, 'd': 1}
  assert letter_count("test") == {'t': 2, 'e': 1, 's': 1}
  assert letter_count("aabbcc") == {'a': 2, 'b': 2, 'c': 2}
  assert letter_count("abc") == {'a': 1, 'b': 1, 'c': 1}
output: def letter_count(s):
      count = {}
      for letter in s:
          if letter in count:
              count[letter] += 1
          else:
              count[letter] = 1
      return count