Update config class check in auto factory

[Rocketknight1]

Our code for custom classes checks that model_class.config_class matches config_class, but this test is behaving oddly - it's returning a failure even though the two classes look identical to me. I changed the test to a string comparison test instead - it's possible that we're just initializing the two classes in slightly different ways, and that's creating two different objects that still represent the same class.

Update: The underlying cause is not my original PR, but PR #29262. The check is failing because we now use importlib.machinery.SourceFileLoader() rather than importlib.import_module, and this causes the two classes to not be equal (even though all their attributes are the same). I think the string comparison test should work fine as a solution for now!

Fixes #29828

(Probably) fixes #30149

[HuggingFaceDocBuilderDev]

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[Rocketknight1]

Pinging @amyeroberts for a quick review here, since we now have two issues complaining about it!

[amyeroberts]

Thanks!

[amyeroberts]

Only concern is that we won't catch things if the attributes aren't the same

[Rocketknight1]

I'm tentatively hopeful based on my testing, but I admit there are some mysteries in the import machinery that I don't fully understand.