input: Write a python function to count the occurrence of each character in a string.
  assert count_chars("Hello World") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'W': 1, 'r': 1, 'd': 1}
  assert count_chars("aabbcc") == {'a': 2, 'b': 2, 'c': 2}
  assert count_chars("abcABC") == {'a': 1, 'b': 1, 'c': 1, 'A': 1, 'B': 1, 'C': 1}
output: def count_chars(s):
      char_count = {}
      for char in s:
          if char in char_count:
              char_count[char] += 1
          else:
              char_count[char] = 1
      return char_count