input: Write a function to replace all occurrences of a given string with an alternate string. The function should take three parameters: the original input string, the target string to be replaced, and the replacement string. The function should return the modified string after performing the replacement.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Hello World!", "o", "a")=="Hella Warld!"
  assert replace_string("Hello World!", "abc", "xyz")=="Hello World!"
  assert replace_string("", "World", "Python")==""
  assert replace_string("Hello World!", "", "Python")=="Hello World!"
output: def replace_string(input_str, target, replacement):
      return input_str.replace(target, replacement)