input: Write a python function to decode a base64 encoded string.
  assert decode_string("SGVsbG8gd29ybGQ=") == "Hello world"
  assert decode_string("UHl0aG9uIGZ1bmN0aW9u") == "Python function"
  assert decode_string("QmFzZTY0IGVuY29kaW5n") == "Base64 encoding"
output: def decode_string(encoded_str):
      import base64
      decoded_bytes = base64.b64decode(encoded_str)
      decoded_str = decoded_bytes.decode('utf-8')
      return decoded_str