input: Write a function to count the number of common substrings between two input strings. If there are no common substrings, return 0.
  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("aaa", "aa")==2
  assert num_common_substrings("", "")==0
output: def num_common_substrings(s1, s2):
      set1 = set(s1[i:j] for i in range(len(s1)) for j in range(i + 1, len(s1) + 1))
      set2 = set(s2[i:j] for i in range(len(s2)) for j in range(i + 1, len(s2) + 1))
      return len(set1 & set2)