fix: resolve issue #13811

[Zhu1116]

What does this PR do?

Fixes #13811

I found an issue in the cond ids processing in train_dreambooth_lora_flux2_img2img.py in diffusers 0.38.0, around lines 1703-1709 of the package. This issue also exists in the main branch.

With the original code:

cond_model_input_list = [cond_model_input[i].unsqueeze(0) for i in range(cond_model_input.shape[0])]
cond_model_input_ids = Flux2Pipeline._prepare_image_ids(cond_model_input_list).to(
    device=cond_model_input.device
)
cond_model_input_ids = cond_model_input_ids.view(
    cond_model_input.shape[0], -1, model_input_ids.shape[-1]
)

When batch size is 2, the output cond_model_input_ids looks like this:

tensor([[[10,  0,  0,  0],
         [10,  0,  1,  0],
         [10,  0,  2,  0],
         ...,
         [10, 31, 29,  0],
         [10, 31, 30,  0],
         [10, 31, 31,  0]],

        [[20,  0,  0,  0],
         [20,  0,  1,  0],
         [20,  0,  2,  0],
         ...,
         [20, 31, 29,  0],
         [20, 31, 30,  0],
         [20, 31, 31,  0]]], device='cuda:0')

However, cond ids within the same batch should not be different.
Flux2Pipeline._prepare_image_ids is designed for multiple conditioning images from the same sample.

With the fixed code:

model_input_ids = Flux2Pipeline._prepare_latent_ids(model_input).to(device=model_input.device)
cond_model_input_ids = Flux2Pipeline._prepare_image_ids([cond_model_input[0:1]]).to(
    device=cond_model_input.device
)
cond_model_input_ids = cond_model_input_ids.expand(
    cond_model_input.shape[0], -1, -1
)

The output becomes correct (same cond ids for the whole batch):

tensor([[[10,  0,  0,  0],
         [10,  0,  1,  0],
         [10,  0,  2,  0],
         ...,
         [10, 31, 29,  0],
         [10, 31, 30,  0],
         [10, 31, 31,  0]],

        [[10,  0,  0,  0],
         [10,  0,  1,  0],
         [10,  0,  2,  0],
         ...,
         [10, 31, 29,  0],
         [10, 31, 30,  0],
         [10, 31, 31,  0]]], device='cuda:0')

I'm sorry if I have misunderstood the underlying logic.

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