[Bugfix] Fix IP-Adapter training

[okotaku]

Motivation

Fix based on huggingface/diffusers#6573.

# encoder_hidden_states should be tuple
encoder_hidden_states, ip_hidden_states = encoder_hidden_states

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

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📚 Documentation preview 📚: https://DiffEngine--131.org.readthedocs.build/en/131/