[Flux] Enabling Flux specific CI

[wwwjn]

Context

1. Add 2 tests for flux specific changes, and setup CI:
   • Integration tests: Running basic, parallelism, checkpointing and other integration tests.
   • Unite tests: currently only one tests to test flux_dataloader.
2. Create a small dataset (sample = 10) for offline CI test environment.

Next step:

• Adding generate image scripts (need basic parallelism), and add a test to generate images.