Fix check for xla in PreTrainedModel.save_pretrained()


# What does this PR do?

Added is_torch_tpu_available() to the condition for saving a model as xla model when calling `PreTrainedModel.save_pretrained()`
The `xla_device` property of `config` can also be `True` on a non-xla device, when loading a checkpoint that was previously trained and saved on xla.

Loading a model that was trained on xla was fixed previously with #5636 , this PR fixes the problem of saving such a model again.

Fixes #7695



## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).
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      Pull Request section?
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests? 


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