fix(checkpoint): restore single-GPU custom-model DCP loading (#3533)

* fix(checkpoint): restore DCP for single-device custom models

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

* Apply suggestion from @akoumpa

* Apply suggestions from code review

Co-authored-by: Alexandros Koumparoulis <153118171+akoumpa@users.noreply.github.com>

* style(checkpoint): format load routing condition

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

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Signed-off-by: HuiyingLi <willwin.lee@gmail.com>
Co-authored-by: Alexandros Koumparoulis <153118171+akoumpa@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>