take care of case when "_tied_weights_keys" is not an attribute

[fabianlim]

What does this PR do?

Address the issue pointed out by @enesmsahin on #3154

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR. @muellerzr @enesmsahin

[HuggingFaceDocBuilderDev]

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[fabianlim]

Its abit strange that the test will fail given its such a small change @BenjaminBossan