Fix GPT-2 warnings
There was a forgotten code path when identifying missing weights.
When loading from a pytorch checkpoint to a tensorflow checkpoint, there was no issue, but doing so the other way around wouldn't check the `_keys_to_ignore_on_load_missing` and `_keys_to_ignore_on_load_unexpected` variables before printing a warning.

closes https://github.com/huggingface/transformers/issues/11192