input: Write a function to replace all occurrences of a given string with an alternate string. The function should take three arguments: the input string, the target string to replace, and the replacement string. If the target string is found in the input string, it should be replaced with the replacement string, otherwise, the input string should remain unchanged.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Hello World!", "Goodbye", "Python")=="Hello World!"
  assert replace_string("Python is fun, Python is cool", "Python", "Java")=="Java is fun, Java is cool"
  assert replace_string("", "anything", "Java")==""
output: def replace_string(input_str, target, replacement):
      return input_str.replace(target, replacement)