Fix target_modules type in config.from_pretrained

[BenjaminBossan]

Fixes #1045, supersedes #1041

Description

When loading a config from a file, we currently set the loaded attributes on the config directly. However, this sidesteps the __post_init__ call, which is required to convert the target_modules to a set. This PR fixes this by avoiding to set attributes on the config class directly, instead of going through __init__.

Other changes

While working on this, I did a slight refactor of the config tests.

1. All config classes are included now (some where missing before).
2. Added a unit test for the aforementioned bug.
3. Use parameterized instead of looping through the classes. Note that this makes the diff look much bigger than it actually is, the tests are actually unchanged, just unindented by one level because the loop is removed.

Notes

This fix is based on my comment here:

#1041 (review)

Normally, I'd just wait for Sourab to reply, but since he's off for a few days, I created a separate PR.

Another way we could achieve this is to override __setattr__ on the config class which explicitly converts target_modules to a set. This would cover the case where a user does something like this:

config = ...
config.target_modules = ["a", "b", "c"]

Using __setattr__, we wouldn't need to rely on __post_init__. However, I would propose to save the heavy guns for when absolutely necessary.

[younesbelkada]

This looks super clean ! Thanks a lot for fixing target_modules type in from_pretrained, adding corresponding tests and extending the config tests for all config classes!