input: Write a function to convert letter grades to GPA values.
  assert convert_letter_grade('A') == 4.0
  assert convert_letter_grade('B') == 3.0
  assert convert_letter_grade('C') == 2.0
  assert convert_letter_grade('D') == 1.0
output: def convert_letter_grade(letter):
      grade_to_gpa = {
          'A': 4.0,
          'B': 3.0,
          'C': 2.0,
          'D': 1.0,
          'F': 0.0
      }
      return grade_to_gpa.get(letter, 0.0)