[`gpt2-int8`] Add gpt2-xl int8 test
# What does this PR do?

Addresses: https://github.com/huggingface/transformers/pull/24504#discussion_r1243813429 

Currently the test is failing because of the following reason (that I need to explore and as discussed offline @sgugger):
1- If a buffer is defined as `persistent=False` in the modeling file
2- and not present in the state_dict
The dispatch_model seems to fail as I face device mismatch issue. I think a potential fix needs to be addressed on accelerate 
Facing the same issue for blip2 int8 tests, as the `cos_cached` and `sin_cached` are defined as buffers with persistent=False and not present in the state dict. 

Seems that the issue does not happen before 5791d949ff93733c102461ba89c8310745a3fa79 on accelerate

cc @sgugger 

EDIT: https://github.com/huggingface/accelerate/pull/1660 should fix the issue I am facing
_The documentation is not available anymore as the PR was closed or merged._<|||||>Can confirm all the tests pass now (with the accelerate PR mentioned above)