input: Write a function to count the number of common substrings between two given strings. The function should return an integer indicating the number of common substrings found between the two strings.
  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", "a")==1
  assert num_common_substrings("", "test")==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)