Add abstractmethod declaration

[mengxunQAQ]

Pull Request check-list

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• Do tests and lints pass with this change?
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• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

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Description of change

Add an abstractmethod declaration to ensure that subclasses do not forget to implement this method.

[petyaslavova]

Hi @mengxunQAQ, thank you for your contribution! We’ll review your change soon.