Improve TransformersModel UX

[hmellor]

Improve user experience by warning instead of raising when Linear layer cannot be tensor parallelised.

(split from #12776)

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[hmellor]

Note that the dtype of the loaded model is handled by a context manager:

vllm/vllm/model_executor/model_loader/loader.py

Lines 381 to 383 in bc1bdec

	with set_default_torch_dtype(model_config.dtype):
	with target_device:
	model = _initialize_model(vllm_config=vllm_config)