[docs] no hard coding cuda as bnb has multi-backend support (#35867)
* change cuda to DEVICE
* Update docs/source/en/llm_tutorial.md
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>