Add custom input policy test

[IonesioJunior]

Description

This PR aims to fix CustomInputPolicy creation and usage workflow and add a new test in custom-policy notebook.

Affected Dependencies

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How has this been tested?

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Checklist

• [ ] I have followed the Contribution Guidelines and Code of Conduct
• [ ] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests

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