input: Write a python function to count the frequency of each letter in a given sentence.
  assert letter_frequency("hello") == {'h': 1, 'e': 1, 'l': 2, 'o': 1}
  assert letter_frequency("aabbbcccc") == {'a': 2, 'b': 3, 'c': 4}
  assert letter_frequency("Test") == {'T': 1, 'e': 1, 's': 1, 't': 1}
output: def letter_frequency(sentence):
      frequency = {}
      for letter in sentence:
          if letter in frequency:
              frequency[letter] += 1
          else:
              frequency[letter] = 1
      return frequency