There tokenizer is missing and when you then call trainer.save_model it only saves the model and not the tokenizer as well.
If you take a look at: https://github.com/huggingface/notebooks/blob/87cde801e765caf5936808c61d4edd62cc32abf1/sagemaker/01_getting_started_pytorch/scripts/train.py#L58
The tokenizer is loaded and passed to the Trainer to save it as well.
Can you share the link where you found this?