[diffusion] CI: send diffusion nightly-test outputs to slack for correctness monitoring

[yhyang201]

Motivation

This PR adds the ability to push diffusion-related performance test outputs to Slack for easier monitoring of generation correctness.
It introduces a new upload_file_to_slack utility and integrates it into the video, image, and image-edit generation flows. For each test case, the generated outputs (and the original image when applicable) are saved temporarily, uploaded to a designated Slack channel along with case ID, model, and prompt metadata, and then cleaned up. This enables clear visualization of test outputs directly in Slack, improving oversight and troubleshooting of generation quality.

Modifications

Accuracy Tests

Benchmarking and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.
• Work with maintainers to merge your PR. See the PR Merge Process

[yhyang201]

/tag-and-rerun-ci

[yhyang201]

Move to #13833