[`GPT2`] Add correct keys on `_keys_to_ignore_on_load_unexpected` on all child classes of `GPT2PreTrainedModel`
# What does this PR do?

as per title

forgot to add them in https://github.com/huggingface/transformers/pull/23256

Currently this snippet:
```python
from transformers import GPT2Model

model = GPT2Model.from_pretrained("gpt2")
```
Gives a big warning:
```bash
Some weights of the model checkpoint at gpt2 were not used when initializing GPT2Model: ['h.10.attn.bias', 'h.5.attn.bias', 'h.7.attn.bias', 'h.0.attn.bias', 'h.11.attn.bias', 'h.8.attn.bias', 'h.1.attn.bias', 'h.9.attn.bias', 'h.2.attn.bias', 'h.4.attn.bias', 'h.6.attn.bias', 'h.3.attn.bias']
- This IS expected if you are initializing GPT2Model from the checkpoint of a model trained on another task or with another architecture (e.g. initializing a BertForSequenceClassification model from a BertForPreTraining model).
- This IS NOT expected if you are initializing GPT2Model from the checkpoint of a model that you expect to be exactly identical (initializing a BertForSequenceClassification model from a BertForSequenceClassification model).
```
This PR fixes it by adding the correct regex expressions on `_keys_to_ignore_on_load_unexpected` for all child classes that inherit from `GPT2PreTrainedModel`

cc @sgugger @patrickvonplaten 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24113). All of your documentation changes will be reflected on that endpoint.