perf: avoid recomputing rotary_emb for each layer in some Google and ModernBERT models

[casinca]

What does this PR do?

Following #45144 (comment)

tl;dr: self.rotary_emb was called N_layers times but only 2 calls are needed (full attn and swa) other calls were overwritting the same dict keys, which was inefficient.
This now gives a little boost depending on config.layer_types size, with the 4B model inference can be ~10% faster (see img below for a benchmark with 300 toks)

See picture below (of the comment #45144 (comment)) for details (comment link will not be directly available after resolve and will just point to the linked PR)

Click to expand image

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It was discussed with @vasqu and also @zucchini-nlp.
I kept Raushan suggestion with just set() (but can also do self.rotary_emb.layer_types it's a minor detail either way.)

[casinca]

I checked gemma4, they are correctly doing:

transformers/src/transformers/models/gemma4/modular_gemma4.py

Line 1236 in 85099df

self.unique_layer_types = set(self.config.layer_types)

But I'll check if some other Google models are also doing it the unoptimized way

[vasqu]

Careful approval, just checking with run-slow + update us if you find other models :D

[vasqu]

run-slow: gemma3

[github-actions]

Workflow Run ⚙️

This comment contains run-slow, running the specified jobs:

models: ["models/gemma3"]
quantizations: []

[casinca]

Careful approval, just checking with run-slow + update us if you find other models :D

Yep, 4 other models and upon looking, it seems safe to extend

Since it's 1 LoC change, I believe it's not worth extra PR noise, I can broaden the changes here + update title? Let me know what you'd prefer, thanks.

[vasqu]

Yes sounds good! Can you wait for the slow tests to return results before pushing? Other than that, feel free to add all of them here + lets adjust the PR title

[github-actions]

CI Results

Workflow Run ⚙️

Commit Info

Context	Commit	Description
RUN	6adc6aa0	workflow commit (merge commit)
PR	67620c54	branch commit (from PR)
main	85099df9	base commit (on main)

✅ No failing test specific to this PR 🎉 👏 !

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gemma3, gemma3n, modernbert, modernbert_decoder, t5gemma2