Support using SigLIP2 text and image embedding as standalone model

[duc-ph]

WIP

[gemini-code-assist]

Code Review

This pull request refactors the SigLIP2 implementation to support using the text and image embedding models as standalone components. The changes are generally in the right direction, but there are several critical issues in the new text model implementation that will cause runtime errors. These include incorrect loop iteration, a method call with missing arguments, and an incorrect call to a superclass constructor. Additionally, there's a potential bug in how vision inputs are checked, which could lead to incorrect behavior for empty inputs. These issues need to be addressed before this PR can be merged.

[github-actions]

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[hmellor]

Closing this PR as the functionality has been superseded by merged work. PR #27324 added SiglipEmbeddingModel which already supports SigLIP2 text and image embeddings as a standalone model.