input: Write a function to count the total number of vowels in a given string. If the given string is empty then return 0.
  assert count_vowels("Hello World")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("AEIOUaeiou")==10
  assert count_vowels("")==0
  assert count_vowels("Bcdfg")==0
output: def count_vowels(s):
      vowels = "AEIOUaeiou"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count