Fix custom client registration

[olgavrou]

Why are these changes needed?

For custom client registration:

• Removing redundant check for placeholder config inside config list
• configs in self._config_list can end up having the extra kwargs passed through (e.g. cache_seed) and therefore the check for placeholder_config in self._config_list will fail

Related issue number

Checks

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• I've added tests (if relevant) corresponding to the changes introduced in this PR.
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[ekzhu]

Can we add a test just to make sure additional config arguments like cache_seed won't trigger an error?

Also in test_custom_client.py (basically all other tests as well, but we don't have to do everything in this PR) we don't need to check for openai package existence as it is a required dependency.

[ekzhu]

Can we add a test just to make sure additional config arguments like cache_seed won't trigger an error?

Also in test_custom_client.py (basically all other tests as well, but we don't have to do everything in this PR) we don't need to check for openai package existence as it is a required dependency.

[olgavrou]

Can we add a test just to make sure additional config arguments like cache_seed won't trigger an error?

Also in test_custom_client.py (basically all other tests as well, but we don't have to do everything in this PR) we don't need to check for openai package existence as it is a required dependency.

@ekzhu good point, added unit test, checked that it failed with old code, and passes with PR changes. Also removed the openai req

[ekzhu]

Remember to check the code in #1667 also to see if it works with this update. It works on my side.