[MISC] add arg pad_for_invariant_seq_len

[MengqingCao]

We need arg pad_for_invariant_seq_len in Ascend NPU backend because variable length input is not supported in FA op of torch-npu yet. Thus this pr ensures the accuracy of the inference on npu.

pad_for_invariant_seq_len is disabled by default, so that it won't influence any other backends.

ps. We'll revert it when the variable length input is supported, if this is not nessary for any other backend

[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.

🚀

[MengqingCao]

@DarkLight1337 could you help review this? Thanks!