input: Write a function to calculate the number of common substrings between two given strings. If there are no common substrings, it should return 0. Otherwise, it should return the count of common substrings.
  
  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("aaaa", "aa")==4
  assert num_common_substrings("", "abc")==0
  
  result = num_common_substrings("abcd", "bae")
  print(result)
output: def num_common_substrings(s1, s2):
      set1 = set()
      set2 = set()
      
      for i in range(len(s1)):
          for j in range(i + 1, len(s1) + 1):
              set1.add(s1[i:j])
      
      for i in range(len(s2)):
          for j in range(i + 1, len(s2) + 1):
              set2.add(s2[i:j])
      
      return len(set1 & set2)