[BE] Use nested namespaces in `torch/csrc/cuda` (#106928)
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### <samp>🤖 Generated by Copilot at 6b1dde1</samp>
> _`namespace` syntax_
> _Simplified with C++17_
> _Code is more readable_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106928
Approved by: https://github.com/huydhn, https://github.com/izaitsevfb
