Add IVON optimizer support

[rockerBOO]

https://arxiv.org/abs/2506.14280
https://github.com/team-approx-bayes/ivon
https://arxiv.org/abs/2402.17641

I'm just trying it out so I dunno if it's good or not.

Requires ivon-opt but only works with my branch because of device, mixed precision, and gradient accumulation issues.

pip install git+https://github.com/rockerBOO/ivon@gradient-accumulation

unet_lr = 1e-4 # Same LR you'd use for Adam
optimizer_type = "ivon.IVON"
optimizer_args = [
    "ess=400" # ess should be the number of items in your dataset. Maybe just the number of items per epoch?
]

enable_pruning=true # Optional, but might improve performance by pruning the high variance values

Default parameters:

        ess: float,
        hess_init: float = 1.0,
        beta1: float = 0.9,
        beta2: float = 0.99999,
        weight_decay: float = 1e-4,
        mc_samples: int = 1,
        hess_approx: str = 'price',
        clip_radius: float = float("inf"),
        sync: bool = False,
        debias: bool = True,
        rescale_lr: bool = True