save_pretrained_model_dir to save_model_dir

In the colab notebook, [`save_pretrained_model_dir`](https://github.com/google/project-euphonia-app/blob/a44b9a3fa28d0d93c1f3db5be980416770cc6736/training_colabs/Project_Euphonia_Finetuning.ipynb#L705) is not defined. The model was saved into [`save_model_dir`](save_model_dir).

BTW, thank you for the project!!