chore: Switch to sha256 from md5 to remedy CodeQL Warning

[kdestin]

Description

This pull request migrates from md5 to sha256

https://github.com/Azure/azureml-examples/blob/6a36786a3f8c8d8e1b1b4cf82f1be966eec842df/sdk/python/jobs/single-step/pytorch/distributed-training-yolov5/yolov5/utils/dataloaders.py#L106

CodeQL warns against weak cryptography. This use of md5 doesn't seem to appear in a cryptographically sensitive context, but the easiest remediation for the CodeQL warnings is to switch to an approved hash function.

Checklist

• [x] I have read the contribution guidelines.
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• [x] I have coordinated with the docs team (mldocs@microsoft.com) if this PR deletes files or changes any file names or file extensions.
• [x] Pull request includes test coverage for the included changes.
• [x] This notebook or file is added to the CODEOWNERS file, pointing to the author or the author's team.