input: Write a function to replace all occurrences of a given string with an alternate string in the input string. The function should take three arguments: the input string, the target string to be replaced, and the replacement string. If the target string is found in the input string, it should be replaced with the replacement string. If the target string is not found, the input string should be returned unchanged.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Good morning!", "morning", "evening")=="Good evening!"
  assert replace_string("Welcome to the future", "future", "present")=="Welcome to the present"
  assert replace_string("Hello World!", "Earth", "Mars")=="Hello World!"
  assert replace_string("", "anything", "nothing")==""
output: def replace_string(input_str, target_str, replacement_str):
      if target_str in input_str:
          return input_str.replace(target_str, replacement_str)
      return input_str