Add checksum to ET to validate file size/contents

[lucylq]

🚀 The feature, motivation and pitch

See internal request here: https://fb.workplace.com/groups/pytorch.edge2.team/permalink/1253025125953337/

Partially-downloaded PTE files may fail when retrieving segments, etc. It's difficult to debug the root cause of these failures.

Adding a checksum will help confirm that the full PTE file has been downloaded at load time before executing.

Alternatives

No response

Additional context

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RFC (Optional)

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[lucylq]

#14226