Remove unnecessary aten::to overhead in llama
As per title, in `LlamaRotaryEmbedding` the [`cos_cached` and `sin_cached` buffers](https://github.com/huggingface/transformers/blob/08ae37c820395e91fc3aa8b801696de5002481d2/src/transformers/models/llama/modeling_llama.py#L94-L104) are not initialized on the right dtype, because `inv_freq` is always initialized on fp32 no matter the default pytorch dtype in use.

The result is that `cos_cached` and `sin_cached` do not obey `torch_dtype=torch.float16` or `torch_dtype=torch.float32`, resulting in unnecessary overheads:

![image](https://github.com/huggingface/transformers/assets/9808326/e1aa5b64-9344-45cf-90be-f3dbf6017e1d)

I leave the `to()` in the `forward`, but it may be removed, WDYT? It is also fine as is, as it is now a no-op:

![image](https://github.com/huggingface/transformers/assets/9808326/83319770-1b7d-4dc6-b7a6-ff056376c1c7)



woops wrong branch<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24202). All of your documentation changes will be reflected on that endpoint.