Improve PEFT UX

[cuichenx]

What does this PR do ?

Two improvements of PEFT UX

1. Correct layer_selection logging messages in the case of more than one adapters.
2. Make output (label, ground truth) field optional for test data, for use cases where user only wants to generate text with gpt_peft_eval and not calculate any metrics.
   • Note that using the default config file, data.test_ds.label_key and data.test_ds.prompt_template still reference an "output" field. These do not need to be changed. This PR simply ignores a key error if data.test_ds.label_key does not exist in the test data file.

Collection: NLP

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[cuichenx]

jenkins

[arendu]

LGTM!