Fix ImportError and SyntaxWarnings

[divyang4481]

This PR addresses two issues reported in the X-Theta framework:

1. An ImportError in scripts/run_open_data_validation.py caused by a missing get_loader function in xtheta/data/loaders.py. I implemented get_loader as a factory function that returns a loader capable of iterating through files and directories.
2. Multiple SyntaxWarning messages in xtheta/visualization/plots.py regarding invalid escape sequences in LaTeX strings used for plot labels and titles. I converted these to raw strings (r'...' or rf'...') to properly handle backslashes.

Verification:

• All existing tests passed (pytest tests/ -v).
• scripts/generate_paper1_artifacts.py runs without warnings.
• scripts/run_open_data_validation.py successfully processed a mock generic dataset.

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PR created automatically by Jules for task 12585272609932296928 started by @divyang4481

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