Correct custom transform documentation

[fepegar]

[Generated by a coding agent]

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Summary

• correct the v2 custom-transform hook names and batch argument types
• explain automatic Subject/SubjectsBatch wrapping and 4D-to-5D tensor changes
• document batched metadata, per-instance behavior, and subject-wise migration
• remove the unsafe direct apply_transform replay example

Validation

• documentation code blocks
• Zensical build
• Ruff documentation lint

Stack (merge bottom-up)

1. Correct custom transform documentation #1493 — documentation corrections (this PR)
2. #1494 — lossless subject batching
3. #1495 — exact transform parameter replay
4. #1496 — subject mapping adapters and examples

[github-actions]

📖 Docs Preview

Preview of the documentation for this PR:

🔗 https://smokeshow.helpmanual.io/4m3c1l0i3269293o6w67/

_{Built from f6c059e}

[Copilot]

Pull request overview

This PR corrects and expands TorchIO v2 documentation around custom transforms and batching, aligning migration and transform-concepts docs with the v2 hook architecture and batched metadata behavior.

Changes:

• Document v2 custom-transform hooks (make_params(batch) / apply_transform(batch, params)) and the automatic Subject↔SubjectsBatch wrapping.
• Add guidance and examples for 4D→5D tensor handling, batched metadata, and per-instance vs shared behavior.
• Remove the unsafe “replay by calling apply_transform directly” example and replace it with safer guidance.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/get-started/migration.md	Adds/clarifies migration guidance for rewriting custom transforms to the v2 batch hook model, including metadata and per-subject escape hatches.
docs/concepts/transforms.md	Updates transform design documentation with a v2-aligned custom transform example and clarifies batching/metadata semantics.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.