TST: Refactor unittest to pytest style custom tests

[BenjaminBossan]

This is a follow up to #2462, #2478, and #2491.

Description

Finish the refactor from unittest-style tests to pytest-style tests to now also include the last big file to still use the old style, test_custom_models.py. This file was already mostly written with pytest in mind, so the changes were rather minimal.

With this class refactored, we can finally remove ClassInstantier, which made understanding test parametrization much more difficult.

There are still a few unittest.TestCase classes found throughout, but they are pretty isolated, so no big need to refactor them right now.

Test coverage

Note that the test coverage is the same except for one difference. For some reason, I found that module level __getattr__ is no longer called when moving from unittest to pytest, such as here:

peft/src/peft/tuners/adalora/__init__.py

Line 32 in 62c9cf3

def __getattr__(name):

This was pretty strange. I found out that these lines are invoked by usage of self.assertWarnsRegex, which is strange, as it appears unrelated. The reason seems to be the unittest code here:

https://github.com/python/cpython/blob/9258f3da9175134d03f2c8c7c7eed223802ad945/Lib/unittest/case.py#L296-L305

Therefore, I think it's safe to ignore this reduction in test coverage.

[HuggingFaceDocBuilderDev]

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[githubnemo]

LGTM :)