allow customized loss function for imputation/informer

[mmo139]

1. Feature description

Add the customized_loss_func feature currently available in SAITS to other imputation models.

2. Motivation

Currently saits_loss_func is used in class _Informer (line 96 of pypots/imputation/informer/core.py), indicating that loss function is MAE and cannot be customized with a model parameter (the original paper used MSE). I adjusted locally for my task for using MSE but it would be nice if customized_loss_func can also be added for informer. Besides, it seems some other models are using saits_loss_func in core.py as well (but I didn't check one by one).

3. Your contribution

I can help with updating core.py / model.py but not sure if rolling out the feature for other models are also desired.

[github-actions]

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