[Misc] Allow passing logits_soft_cap for xformers backend

[Isotr0py]

• Similar to [Bugfix][Hardware][CPU] Enable Gemma2 with SDPA on CPU backend #11169, allow passing logits_soft_cap to enable Gemma2 inference for xformers backend.

"The Gemma 2 team observed very minor differences when soft-capping is removed during inference." (https://huggingface.co/blog/gemma2#soft-capping-and-attention-implementations)

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀