Fix torchscript tests for GPT-NeoX
# What does this PR do?

Fix torchscript tests for GPT-NeoX. The main issue comes from the fact that current `RotaryEmbedding` changes the model structure in `forward`.

This PR creates the necessary embeddings in `__init__`, which basically makes the cache (of embedding) mechanism useless. Furthermore, the attribute names seems a bit confusing now. We could probably add some attribute (ex. `init_sin_cos_cache_seq_len`) in config with a value `<= max_position_embeddings`, but I think it's way too much.

Not certain if it is worth it. However, with a PR opened, we have a reference.

The current failing test is 
https://github.com/huggingface/transformers/runs/7216768053?check_suite_focus=true
_The documentation is not available anymore as the PR was closed or merged._<|||||>> LGTM!
> 
> However, could we add the failing test for reference or do we need to add a new test here?

I updated the PR description to include the current failing test. Regarding new tests, I don't think it's necessary, as we just build the necessary tensors in `__init__` instead of in `forward`, and the current set of tests should be enough :-)

(however, let me know if you have some idea of new necessary test cases!)<|||||>> > LGTM!
> > However, could we add the failing test for reference or do we need to add a new test here?
> 
> I updated the PR description to include the current failing test. Regarding new tests, I don't think it's necessary, as we just build the necessary tensors in `__init__` instead of in `forward`, and the current set of tests should be enough :-)
> 
> (however, let me know if you have some idea of new necessary test cases!)

Perfect thanks!