[DreamBooth SDXL LoRA] Apply SNR min-weighting to prior preservation loss when snr_gamma is set

[Dev-X25874]

What does this PR do?

In examples/dreambooth/train_dreambooth_lora_sdxl.py, when training with
both --snr_gamma and --with_prior_preservation, the Min-SNR loss
weighting (Section 3.4 of https://huggingface.co/papers/2303.09556) is
applied to the instance loss but silently skipped for the prior preservation
loss. The prior loss is computed before the snr_gamma branch runs, so
mse_loss_weights is never multiplied into it — the prior loss always
falls back to plain MSE regardless of --snr_gamma.

This PR fixes the snr_gamma branch to also apply mse_loss_weights to
prior_loss, mirroring the same pattern already used for the instance loss.
A new test covering the --snr_gamma + --with_prior_preservation combination
is also added to test_dreambooth_lora.py.

Fixes # (issue)

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Who can review?

@sayakpaul