input: Write a function to count the number of common substrings between two given strings. If no common substrings are found, 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("abc", "def")==0
  assert num_common_substrings("aaa", "aaa")==3
  assert num_common_substrings("", "abc")==0
  assert num_common_substrings("abc", "")==0
output: def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              k = 0
              while i + k < len(s1) and j + k < len(s2) and s1[i + k] == s2[j + k]:
                  k += 1
                  count += 1
      return count