[BugFix] Fix sparse NCCL weight transfer test construction

[bedeks]

Purpose

Fix #44272 forward for the tests that failed in the nightly CI.

The production path already passes the model through WeightTransferEngineFactory.create_engine(...); the breakage was in stale test/example-style call sites that still constructed NCCLWeightTransferEngine(config, parallel_config) directly which run only when >2 GPUs.

This updates:

tests/distributed/test_weight_transfer.py
docs/training/weight_transfer/base.md

Test Plan

• Ran targeted sparse NCCL regression tests on a two-GPU L40 environment.
• Ran related sparse worker/model-runner tests.
• Ran the full distributed weight transfer suite covering NCCL dense, NCCL sparse, and IPC paths.
• Ran the sparse NCCL example to validate the real-model dense-vs-sparse update flow.

Test Result

• Targeted sparse NCCL regression tests passed.
• Related sparse worker/model-runner tests passed.
• Full distributed weight transfer suite passed.
• Sparse NCCL example completed successfully with matching dense and sparse update behavior.

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[mergify]

Documentation preview: https://vllm--44345.org.readthedocs.build/en/44345/