Fix test fetcher

[ydshieh]

What does this PR do?

After #35167, the __init__ under each model directory don't import stuff from the python files inside it. The test fetcher has since trouble to get the right dependencies, and our CircleCI is sometimes not running any actual tests.

This PR uses define_import_structure to fix the issue

[HuggingFaceDocBuilderDev]

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[ArthurZucker]

LGTM but lysandre is more familiar with this!

[LysandreJik]

Thank you Yih-Dr!

[ydshieh]

There is one failure

tests/repo_utils/test_tests_fetcher.py::TestFetcherTester::test_create_reverse_dependency_map - ValueError:KeyError:src/transformers/models/bert/tokenization_bert.py. From src/transformers/models/bert/init.py

I will adjust the test and merge.