[Fix] Adjust average model and make EMA in line with MMGen

[plyfager]

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Motivation

In MMGen, we use momentum* EMA + (1 - momentum) * SRC for updating model weights.

Modification

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[LeoXing1996]

I think we can add an argument called decay, and change momentum in our configs to decay.

Refers to: https://github.com/huggingface/diffusers/blob/4125756e88e82370c197fecf28e9f0b4d7eee6c3/src/diffusers/training_utils.py#L84-L94

[zengyh1900]

You can also refer to https://github.dev/open-mmlab/mmengine/blob/main/mmengine/model/averaged_model.py#L33
which we should align the design and parameter conversation.

[zengyh1900]

#1581