fused_backward_pass in prodigy-plus-schedule-free [SOLVED VIA EXTERNAL SOLUTION ✅]

Hi, Kohya. I know you hardcoded fused_backward_pass to Adafactor, but prodigy-plus-schedule-free https://github.com/LoganBooker/prodigy-plus-schedule-free has that feature inside already, but we cant use it. That is, in fact, we can apply the built-in argument itself, but it breaks the training process. Can you add more flexibility in this case, please?

Start by reviewing the issue description and the linked prodigy-plus-schedule-free project, then investigate how its fused_backward_pass feature interacts with the built-in argument. The issue does not name files, tests, reproduction steps, or a precise acceptance condition; completion would require defining those and confirming training no longer breaks.

Written by the indexing model from the issue text.