src/transformers/trainer.py/Trainer/_load_from_checkpoint
class Trainer:    def _load_from_checkpoint(self, resume_from_checkpoint, model=None):
        if model is None:
            model = self.model

        if not os.path.isfile(os.path.join(resume_from_checkpoint, WEIGHTS_NAME)) and not os.path.isfile(
            os.path.join(resume_from_checkpoint, WEIGHTS_INDEX_NAME)
        ):
            raise ValueError(f"Can't find a valid checkpoint at {resume_from_checkpoint}")

        logger.info(f"Loading model from {resume_from_checkpoint}.")

        if os.path.isfile(os.path.join(resume_from_checkpoint, CONFIG_NAME)):
            config = PretrainedConfig.from_json_file(os.path.join(resume_from_checkpoint, CONFIG_NAME))
            checkpoint_version = config.transformers_version
            if checkpoint_version is not None and checkpoint_version != __version__:
                logger.warning(
                    f"You are resuming training from a checkpoint trained with {checkpoint_version} of "
                    f"Transformers but your current version is {__version__}. This is not recommended and could "
                    "yield to errors or unwanted behaviors."
                )

        if os.path.isfile(os.path.join(resume_from_checkpoint, WEIGHTS_NAME)):
            # If the model is on the GPU, it still works!
            if is_sagemaker_mp_enabled():
                if os.path.isfile(os.path.join(resume_from_checkpoint, "user_content.pt")):
                    # If the 'user_content.pt' file exists, load with the new smp api.
                    # Checkpoint must have been saved with the new smp api.
                    smp.resume_from_checkpoint(
                        path=resume_from_checkpoint, tag=WEIGHTS_NAME, partial=False, load_optimizer=False
                    )
                else:
                    # If the 'user_content.pt' file does NOT exist, load with the old smp api.
                    # Checkpoint must have been saved with the old smp api.
                    if hasattr(self.args, "fp16") and self.args.fp16 is True:
                        logger.warning(
                            "Enabling FP16 and loading from smp < 1.10 checkpoint together is not suppported."
                        )
                    state_dict = torch.load(os.path.join(resume_from_checkpoint, WEIGHTS_NAME), map_location="cpu")
                    # Required for smp to not auto-translate state_dict from hf to smp (is already smp).
                    state_dict["_smp_is_partial"] = False
                    load_result = model.load_state_dict(state_dict, strict=True)
                    # release memory
                    del state_dict
            else:
                # We load the model state dict on the CPU to avoid an OOM error.
                state_dict = torch.load(os.path.join(resume_from_checkpoint, WEIGHTS_NAME), map_location="cpu")
                # workaround for FSDP bug https://github.com/pytorch/pytorch/issues/82963
                # which takes *args instead of **kwargs
                load_result = model.load_state_dict(state_dict, False)
                # release memory
                del state_dict
                self._issue_warnings_after_load(load_result)
        else:
            # We load the sharded checkpoint
            load_result = load_sharded_checkpoint(model, resume_from_checkpoint, strict=is_sagemaker_mp_enabled())
            if not is_sagemaker_mp_enabled():
                self._issue_warnings_after_load(load_result)