Fix(research/delf): Replace os.system with native Python libs in dataset_download.py

[lucasbazan]

Description

Refactored research/delf/delf/python/datasets/sfm120k/dataset_download.py to remove dependencies on OS-specific shell commands invoked via os.system.

Changes implemented:

1. Download: Replaced os.system('wget ...') with urllib.request.urlretrieve.
2. Extraction: Replaced os.system('tar ...') with the native tarfile module.
3. Cleanup: Replaced os.system('rm ...') with os.remove.
4. Symlinks: Replaced os.system('ln -s ...') with os.symlink.

Motivation and Context

• Portability: The previous implementation failed on Windows environments because it relied on wget, rm, and ln binaries which are not present by default. Using native Python libraries ensures cross-platform compatibility.
• Security: Removing os.system eliminates the risk of shell injection vulnerabilities and is considered a best practice for file operations.

Dependencies

• No new external dependencies. Uses standard libraries: urllib, tarfile, os, shutil.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update
• TensorFlow 2 migration
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• A new research paper code implementation
• Other (Specify)

Tests

I manually verified the changes by running the script locally:

1. Verified that the dataset files are downloaded correctly via urllib.
2. Verified that tarfile correctly extracts the archives to the target directory.
3. Verified that symlinks are created successfully (on systems that support it).

Checklist

• I have signed the Contributor License Agreement.
• I have read guidelines for pull request.
• My code follows the coding guidelines.
• I have performed a self code review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.

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