Adding backwards optimization of initial conditions as a trainer class

[jsschreck]

Keeping it minimal for now.

I will add a documentation page eventually for this, but I wanted to get it in front of eyes other than mine to double check that I have it implemented correctly according to Hakim/Vonich paper https://arxiv.org/pdf/2504.20238.

[djgagne]

From the CI check:

tests/test_models.py:8: in <module>
    from credit.models import load_model
credit/models/__init__.py:6: in <module>
    from credit.models.crossformer import CrossFormer
credit/models/crossformer.py:12: in <module>
    from credit.models.unet_attention_modules import load_unet_attention
E   ModuleNotFoundError: No module named 'credit.models.unet_attention_modules'
=========================== short test summary info ============================
ERROR tests/test_models.py