[refactor] Pooling layers

[hglee98]

Description

This PR aims to replace pooling layers into custom Pool blocks.

Closes: N/A

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Change(s)

• Add POOL_REGISTRY dict
• Add Pool class into models.op.custom
• Replace with custom Pool implementation

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[hglee98]

Thank you for the review! I've included your feedback from additional commits!🙇🏻‍♂️