input: Write a function to count the number of common substrings between two given strings. The function should return an integer indicating the count of common substrings.
  assert num_common_substrings("abcd", "bae")==2
  assert num_common_substrings("abc", "cba")==3
  assert num_common_substrings("hello", "world")==1
  assert num_common_substrings("xyz", "abc")==0
  assert num_common_substrings("", "")==0
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)