input: Write a function that takes in two strings and returns an integer indicating the number of common substrings between the two strings. The function should iterate through each character of both strings and count how many characters are common between them. If there are no common substrings, the function should return 0.
  
  def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              if s1[i] == s2[j]:
                  count += 1
      return count
  
  assert num_common_substrings("abcd", "bae")==2
  assert num_common_substrings("hello", "world")==1
  assert num_common_substrings("abc", "def")==0
  assert num_common_substrings("", "xyz")==0
  assert num_common_substrings("python", "typ")==2
output: def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              if s1[i] == s2[j]:
                  count += 1
      return count