Change `torch_dtype` to str when `saved_model=True` in `save_pretrained` for TF models
# What does this PR do?

One issue in #22731 is: the config contains `torch_type` with a torch dtype class as value. Usually, our `save_pretrained` will take care of it. But when `saved_model=True` in `save_pretrained` for TF models, it is not handled, and TF/Keras complains about it.

See the first part in [this comment](https://github.com/huggingface/transformers/issues/22731#issuecomment-1506538658)
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