🧞 Add output_layer to the list of lm_head_namings in AutoModelForCausalLMWithValueHead

[qgallouedec]

What does this PR do?

Allows

from trl import AutoModelForCausalLMWithValueHead

model = AutoModelForCausalLMWithValueHead.from_pretrained("THUDM/chatglm3-6b-128k", trust_remote_code=True)

Fixes #2299

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