[Fix Bugs] Fix keys in `_load_pretrained_model`

[hanrui1sensetime]

What does this PR do?

Fixes the bug when _load_pretrained_model. f'{prefix}.key' is wrong because the variable key is not used is this branch case. And this bug will lead to load some models failed like BLOOM-176B.

Before submitting

• [x] 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.
• [x] 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.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[amyeroberts]

cc @sgugger