[BE] Use nested namespaces in sparse

[malfet]

🤖 Generated by Copilot at 59a5205

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

[pytorch-bot]

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❌ 1 Failures

As of commit 63394b7:

NEW FAILURES - The following jobs have failed:

• linux-bionic-cuda11.8-py3.10-gcc7 / test (default, 3, 5, linux.4xlarge.nvidia.gpu) (gh)

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[malfet]

@pytorchbot merge

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[pytorchmergebot]

Merge failed

Reason: 1 jobs have failed, first few of them are: trunk / linux-bionic-cuda11.8-py3.10-gcc7 / test (default, 3, 5, linux.4xlarge.nvidia.gpu)

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[malfet]

@pytorchbot merge -f "TestTorch::test_typed_storage_deprecation_warning is due to trunk regressions, perhaps fixed by #97628 "

[pytorchmergebot]

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