[WIP][BREAKING][worker, ckpt] support checkpoint engine for sync parameters in hybrid mode

[kip-cxj]

What does this PR do?

Implement the checkpoint engine as a standalone update weights module for hybrid mode.

Currently, the inference backend does not support vllm. Adaptation for the vLLM module requires separating actor and rollout into separate processes:#4280

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  1. [BREAKING][recipe, ckpt] feat: support parameter sync by checkpoint-engine. only for fully_async mode. #4427
  2. [wip][BREAKING][recipe, ckpt]add checkpoint engine for one step off policy #4601
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[gemini-code-assist]

Code Review

This pull request introduces a checkpoint_engine to handle weight synchronization in hybrid mode, with changes across FSDP and Megatron workers, as well as vLLM and SGLang rollouts. The implementation adds new configuration flags and logic to conditionally use this new engine.

My review has identified a few critical issues:

1. A potential bug in fsdp_workers.py where incorrect weights might be updated due to a variable mix-up. This is coupled with significant code duplication that should be refactored.
2. Unsafe access to environment variables in vllm_rollout.py which could lead to worker crashes if the environment is not perfectly configured.

I've provided code suggestions to address these critical issues for improved correctness and robustness.

[gemini-code-assist]

Accessing environment variables using os.environ["KEY"] is unsafe as it will raise a KeyError if the variable is not set, causing the worker to crash. It's much safer to use os.getenv("KEY", default_value).

This is a critical issue that can lead to runtime crashes if the environment is not perfectly configured.

Suggested change

	rank = int(os.environ["RANK"])
	local_world_size = int(os.environ["RAY_LOCAL_WORLD_SIZE"])
	rank = int(os.getenv("RANK", "0"))
	local_world_size = int(os.getenv("RAY_LOCAL_WORLD_SIZE", "1"))

[Cesilina]

Timeout error?

[kip-cxj]

Timeout error?

What backend is used for rollout? For vLLM, it's not supported because the actor and rollout still share the same process.

[Cesilina]

Timeout error?

What backend is used for rollout? For vLLM, it's not supported because the actor and rollout still share the same process.

Oo.Get it.Thanks