[BE] Use nested namespaces in sparse (#97581)
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### <samp>🤖 Generated by Copilot at 59a5205</samp>
This pull request refactors the namespace declarations in several files under `aten/src/ATen/native/sparse` to use a more concise and consistent syntax. This improves the readability and reusability of the sparse tensor operations code.
Also, do not rely on deprecated `TensorBase::data` and instead use `TensorBase::data_ptr`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97581
Approved by: https://github.com/kit1980, https://github.com/huydhn
