chore: add tests for flux speed

[kungtalon]

Motivation

Add a new test under tests/flux for automated checking of the pipeline latency

Modifications

1. Added diffusers in requirements.txt
2. Added a speed test for flux models and tested on both RTX 40- and 50- gpus

Checklist

• Code is formatted using Pre-Commit hooks.
• Relevant unit tests are added in the tests directory following the guidance in Contribution Guide.
• Documentation and example scripts in examples are updated if necessary.
• Throughput/latency benchmarks and quality evaluations are included where applicable.
• For reviewers: If you're only helping merge the main branch and haven't contributed code to this PR, please remove yourself as a co-author when merging.
• Please feel free to join our Slack, Discord or WeChat to discuss your PR.

[lmxyy]

Passed all tests. Approved.