input: Write a function to find the start index of a substring in a given string. If the substring is not found in the string, return -1.
  assert find_substring("HelloWorld", "World")==5
  assert find_substring("HelloWorld", "Hello")==0
  assert find_substring("HelloWorld", "oWo")==4
  assert find_substring("HelloWorld", "Earth")==-1
  assert find_substring("", "Hello")==-1
output: def find_substring(s, sub):
      index = s.find(sub)
      return index