input: Write a function to count the number of common substrings between two given strings. If there are no common substrings, return 0. The function should return an integer.
  assert num_common_substrings("abcd", "bae")==2
  assert num_common_substrings("abcdef", "ghijk")==0
  assert num_common_substrings("xyz", "xyx")==3
  assert num_common_substrings("abc", "abc")==3
  assert num_common_substrings("", "any")==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)