Ignore `past_key_values` during GPT-Neo inference
# What does this PR do?

Applies #8633 to GPT-Neo.

I was getting errors like `RuntimeError: Sizes of tensors must match except in dimension 2. Got 20 and 19 (The offending index is 0)` during the evaluation step of `Trainer.train()` with GPT-Neo.

This was fixed in an internal implementation of `GPTNeoForSequenceClassification` by @manuelciosici before an official version of the class was released for `transformers`. I converted our code to use the `transformers` version of the class and realized the solution should be upstreamed. I'm just mentioning this here because I don't get all the credit for this patch.

This might be needed for other models as well, but I don't know of a good way to figure that out without testing them all.

## Who can review?

trainer: @sgugger