Merging latest code update from kohya

[bmaltais]

- Merging latest code update from kohya
- Added `--max_train_epochs` and `--max_data_loader_n_workers` option for each training script.
- If you specify the number of training epochs with `--max_train_epochs`, the number of steps is calculated from the number of epochs automatically.
- You can set the number of workers for DataLoader with `--max_data_loader_n_workers`, default is 8. The lower number may reduce the main memory usage and the time between epochs, but may cause slower dataloading (training).
- Fix loading some VAE or .safetensors as VAE is failed for `--vae` option. Thanks to Fannovel16!
- Add negative prompt scaling for `gen_img_diffusers.py` You can set another conditioning scale to the negative prompt with `--negative_scale` option, and `--nl` option for the prompt. Thanks to laksjdjf!
- Refactoring of GUI code and fixing mismatch... and possibly introducing bugs...