[rl] Using JobConfig as the centralized config system for inference and simple GRPO

[wwwjn]

1. Add job_config.py to extend current JobConfig. Now an issue is trainer's config and generator's config are not symmetric, eg Parallelism and Generation.parallelism
2. Use job config system as the centralized / source-of-truth config, loading config from run_configs/qwen3_0.6b.toml file.
3. Refactor the generator to use EngineArgs() and LLMEngine(), instead of LLM()
4. Rename simple_rl_multiprocess -> simple_grpo to be more descriptive
5. Clean up unused code branch

Test: (trainer ddp = 2, n_generator =1)

Following-up refactors:

• Refactor2: vllm model register - using setup.py and plugin instead of import
• Refactor3: Weight updater, by directly passing state_dict (DTensor) between trainer and generator
• Refactor4: Use torchtitan Trainer, modularize each component

[wwwjn]

We really need a better name for this field, as well as RL

[wwwjn]

Close with #2191