refactor(core): change GGSW iterators to match the decomposition logic

[agnesLeroy]

Resolves: https://github.com/zama-ai/concrete-core-internal/issues/68

Description

Checklist

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• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)
• [ ] The PR description links to the related issue (to link an issue, use '#XXX'.)
• [ ] The tests on AWS have been launched and are successful (comment with @slab-ci cpu_test and/or @slab-ci gpu_test to trigger the tests)
• [ ] The draft release description has been updated
• [ ] Check for breaking changes (including serialization changes) and add them to commit message following the conventional commit specification