Add tests using mocker to main.py

[sweep-nightly]

Description

This PR adds tests using mocker to the main.py file in order to test the data loading and preprocessing steps.

Summary of Changes

• Created a new file named test_main.py in the tests directory.
• Imported the necessary modules for testing, including pytest, mocker, and the modules used in main.py (PIL, torch, torchvision, numpy).
• Added test functions for the data loading and preprocessing steps.
• Used mocker to mock the necessary functions and checked that they are called with the correct arguments.
• Added docstrings and comments to explain the purpose and logic of each test.

Please review and merge this PR. Thank you!

Fixes #10.

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• Rollback changes to tests/test_main.py

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