Several fixes related to rotary position embeddings
Author: mseeger
Pull request: yes

# What does this PR do?

- Fixes bug if rotary_ndims or head_size odd
- Changes related to `position_embeddings` being a mandatory argument
- Remove `position_ids` argument of `apply_rotary_pos_emb`
- Replace `torch.stack` by `torch.cat`, former requires equal shapes

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Fixes # 35233


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@Rocketknight1
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Comments:
@Rocketknight1 , @ArthurZucker : This is the PR which replaces #35376 . This here fixes issue #35233.

This PR does not change behavior, meaning that for some models, the config parameter `partial_rotary_factor` is present, but is not used in the code. I've marked this with TODO. In these cases, trying to set `partial_rotary_factor` to a value different from 1 is refused.


One side comment. I did a fair bit of open source PRs, and this one has been one of the hardest judging the amount of work. It seems copy and pasting code is somehow your trademark, but this makes it very hard to fix something in the code that is copied many times (but even this code has 5 different versions, in the case of RoPE).

You seem to have some tooling, at least to check all these copies (which makes "make fixup" a tough exercise). Maybe you should write some dev guide how to efficiently use this tooling, so that somebody like me just has to change some "master files" and then all the copies are done automatically?


The test which fails, is flaky. You cannot compare such small timing numbers, there is noise on them.

@ArthurZucker This PR seems big, but in essence just because of the coy&paste structure of the codebase.

One exception is the `esm` model, where I fixed something. The current code is just wrong, it ignores the fact that the `cos` and `sin` values depend on `position_ids`.

Let me know whether you want this to be split out. And if so, whether you want that before or after this one here.

@ArthurZucker Anything I can still do here?

Anything I should still do there? This fixes a bug. It also cleans up things a bit.

It would be nice to move forward with this one. It fixes a bug and cleans things up.