[mzeynali]

What does this PR do?

Fixes text conditioning leakage in SDXL InstructPix2Pix training. When a prompt was dropped, encoder_hidden_states was replaced with empty-prompt conditioning, but add_text_embeds still contained the original prompt's pooled embedding.

This change also replaces the pooled embedding with the second text encoder's empty-prompt output, using the same dropout mask.

Validation

Added a regression test using tiny, real SDXL components and local synthetic data:

• With full text dropout, changing only the prompt produces identical trained UNet weights.
• Without dropout, changing the prompt affects the trained weights.

Both cases pass with the fix. The full-dropout case fails on the original code.

Lint, formatting, and check_ai.py passed. make style and make fix-copies passed in an isolated snapshot. GPU training was not tested.

AI-assisted self-review

Reviewed against the repository guidelines. No blocking findings or dead code were identified, and no findings were intentionally left unresolved.