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

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@muellerzr @enesmsahin

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[fabianlim]

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

[BenjaminBossan]

Thanks for the small fix.

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

Yes, I'm pretty sure it's unrelated. I checked out your change locally and the same test passed for me. Also, I see that other PRs are also having issues with this test. Thus we can just ignore it.