Fix AttributeError in Flux2 dreambooth: rename --validation_image to --validation_image_path

[arrdel]

What does this PR do?

Fixes #12779

This PR fixes an error in the Flux2 dreambooth training script.

The Problem

The argument parser defined the argument as (line 358):

parser.add_argument(
    "--validation_image",
    type=str,
    default=None,
    help="path to an image that is used during validation as the condition image to verify that the model is learning.",
)

However, the code on line 1422 references :

validation_image = load_image(args.validation_image_path).convert("RGB")

This naming mismatch causes an when the script tries to access the non-existent attribute.

The Solution

Renamed the argument from to to match what the code expects.

Testing

The fix ensures that:

• ✅ The argument can be properly parsed
• ✅ The code can access without error
• ✅ The script no longer crashes with AttributeError when using validation images