Flux2 Edit training

[dxqb]

Install:

git branch -D pr-1301
git fetch upstream pull/1301/head:pr-1301
git switch pr-1301

this PR uses the existing dataloader infrastructure for inpainting models in OneTrainer. because of that, there are a few limitations:

• only 1 reference image
• reference image filename is hardcoded to filename-condlabel.png when your training sample is filename.jpg. Note the png.
• if enabled, there must be a reference image for all images
• the reference image should have the same resolution as the training sample
• edge case: don't use together with masked training when Unmasked Probability > 0