optimize: eliminate duplicate split_enc_dec_inputs calls

[nicole-lihui]

Purpose

eliminate duplicate split_enc_dec_inputs calls

This improves performance by
reducing redundant function calls while maintaining the same validation logic.

Test Plan

Test Result

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @nicole-lihui.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[gemini-code-assist]

Code Review

This pull request refactors the code to eliminate duplicate calls to split_enc_dec_inputs in llm_engine.py and processor.py. By calling split_enc_dec_inputs once and passing the results to _validate_model_inputs, the change improves performance by reducing redundant work. The implementation is correct and maintains the existing validation logic. I have reviewed the changes and found no issues.

[DarkLight1337]

Thanks for simplifying!