Add missing module __spec__
Author: laurahanu
Pull request: yes

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This PR adds a missing `__spec__` object when importing the library that would be `None` otherwise.
Fixes #12904

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Comments:
Thanks for your PR!

As you can see, this makes all the tests fail because you changed the init of `_LazyModule` without adapting all the places it's used (all the intermediates init of each model). I'm not sure whether those intermediate inits need to pass along the spec attribute or not, if they do you should add it in each one of them (don't forget the model template as well), and if they don't, you should make that argument optional.

@sgugger Thanks for looking at it! Changed the `module_spec` arg to be optional as I don't see why the other intermediate inits would need it.

Great! One last thing: could you run `make style` on your branch to solve the code quality issue?

Last thing caught by the CI new that the style is correct: your new test file will never be run by the CI. Since it's linked to `_LazyModule` defined in file_utils, could you move it to `test_file_utils`? Thanks a lot.

Is this one ready to be merged and published?