[CLAP] Replace hard-coded batch size to enable dynamic ONNX export
Author: xenova
Pull request: yes

# What does this PR do?

This PR unblocks adding ONNX export support for CLAP models in Optimum (see [PR](https://github.com/huggingface/optimum/pull/1552)), by replacing hard-coded batch size in the `window_reverse` function. This was [already done in the past for other similar models](https://github.com/huggingface/transformers/commit/b651efe59ea506d38173e3a60a4228e7e74719f9) (by @lewtun), but since then, [CLAP was added](https://github.com/huggingface/transformers/pull/21370) (by @ArthurZucker), and this change was not brought across there.

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Fixes https://github.com/huggingface/optimum/pull/1552#issuecomment-1836233113


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_27790). All of your documentation changes will be reflected on that endpoint.

Added back the docstring 👍 - ready to merge imo? :) 

Yep just make style and rebase should do the trick for the CI

Done 👍 (failed style seemed to be unrelated; should be fixed now)
![image](https://github.com/huggingface/transformers/assets/26504141/86542daf-25a4-4873-9fbb-539323bf92f7)


Thanks 👍🏻