[BugFix] NetLoader: No backend type associated with device type npu

[destinysky]

What this PR does / why we need it?
This PR fixes a bug in NetLoader PR#2888. The bug was caused by PR#3612 ([1/N][Refactor] Refactor code to adapt with vllm main), which removed the stateless_init_device_torch_dist_pg function from platform.py, leading to a failure in the call. This PR adds a way to create a stateless process group that does not depend on external code.

Does this PR introduce any user-facing change?
No

How was this patch tested?
Same with PR#2888

• vLLM version: v0.13.0
• vLLM main: vllm-project/vllm@2f4e654

[github-actions]

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[gemini-code-assist]

Code Review

This pull request addresses a bug in NetLoader caused by a previous refactoring that removed necessary process group initialization logic. The fix introduces a new file, vllm_ascend/model_loader/netloader/executor/netloader_pg.py, which contains self-contained functions (stateless_init_process_group and destroy_stateless_process_group) for managing stateless HCCL process groups on NPU devices. The changes in elastic_load.py simply adopt these new utility functions. My review focuses on the new implementation in netloader_pg.py, where I've found a couple of areas for improvement regarding code correctness and robustness.