fix: implement lazy imports for BetterTransformer

[aryasaatvik]

Related Issue

Fixes import errors when using --no-bettertransformer flag with transformers >= 4.49.

Summary

This PR implements lazy imports for BetterTransformer to prevent import errors when it's disabled or when using incompatible transformers versions. BetterTransformer is deprecated in optimum and requires transformers < 4.49.

Changes:

• Replaced eager imports with lazy loading pattern
• Added _import_bettertransformer() function for on-demand imports
• Wrapped imports in try/except to handle version conflicts gracefully
• Returns early when --no-bettertransformer is used, avoiding unnecessary imports

Why this is needed:

• BetterTransformer is deprecated and incompatible with transformers >= 4.49
• Users with modern transformers versions get import errors even with --no-bettertransformer
• This allows the flag to work as intended

Checklist

• [x] I have read the CONTRIBUTING guidelines.
• [ ] I have added tests to cover my changes.
• [ ] I have updated the documentation (docs folder) accordingly.

Additional Notes

This change ensures backward compatibility for users who still want BetterTransformer while fixing the issue for users with newer dependencies.

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Codecov Report

:x: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 79.54%. Comparing base (f84d8e2) to head (947044d).

Files with missing lines	Patch %	Lines
...inity_emb/infinity_emb/transformer/acceleration.py	72.22%	5 Missing :warning:
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #619   +/-   ##
=======================================
  Coverage   79.54%   79.54%           
=======================================
  Files          43       43           
  Lines        3486     3501   +15     
=======================================
+ Hits         2773     2785   +12     
- Misses        713      716    +3     

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[wirthual]

Absolutely. According to this issue:

please use transformers' attention implementation: https://huggingface.co/docs/transformers/main/en/llm_optims#attention and torch.compile (with static cache if decoder): https://huggingface.co/docs/transformers/main/en/llm_optims#static-kv-cache-and-torchcompile for the best possible performance (exceeding bettertransformer, which no one maintains! 💀).

What are your thoughts on this method vs continue support for current bettertransformer?

[wirthual]

@michaelfeil One option to keep bettertransformer as was is #641. Extracted bettertransformer code in its own package. This should get rid of version error while keeping the orig bettertransformer code. Not sure if/how long this method can work.