refactor: postprocessors/register.py -> registry.py

[aychun]

Description

Change the file name of post-processor registry to match with others.

Closes: #423

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Change(s)

• postprocessors/register.py -> postprocessors/registry.py

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- Added a new feature by `@myusername` in [PR 2023](https://github.com/Nota-NetsPresso/netspresso-trainer/pull/2023)

[illian01]

Thank you for your pull request! It looks good to me.
I'm going to update CHANGELOG.md. Can you activate allow edits and access to secrets by maintainers button?