Add load_optim option to load checkpoint but not optimizer state (#229)

                self.lr_scheduler, self._num_updates, self._optim_history, extra_state,

            )

    def load_checkpoint(self, filename):

    def load_checkpoint(self, filename, load_optim=True):

        """Load all training state from a checkpoint file."""

        extra_state, self._optim_history, last_optim_state = \

        extra_state, optim_history, last_optim_state = \

            utils.load_model_state(filename, self.model)

        if last_optim_state is not None:

            # rebuild optimizer after loading model, since params may have changed

            self._build_optimizer()

            if load_optim:

                self._optim_history = optim_history

                # only reload optimizer and lr_scheduler if they match

                last_optim = self._optim_history[-1]

                if last_optim['criterion_name'] == self.criterion.__class__.__name__: