input: Write a python function to find the greatest common divisor (GCD) of two numbers.
  assert find_gcd(48, 18) == 6
  assert find_gcd(100, 25) == 25
  assert find_gcd(7, 3) == 1
output: def find_gcd(a, b):
      while b:
          a, b = b, a % b
      return a