Enabling users to customize loss function

[WenjieDu]

1. Feature description

Add a feature to enable users to specify their own loss functions, which should be callable python functions.

2. Motivation

Currently the loss functions in PyPOTS models are fixed. This definitely has no problem because reproducing algorithms and models exactly is an important part in PyPOTS project. Algorithms and models should be kept as same as possible with the descriptions from original papers.

However, users from time to time have to specify the loss function for better optimization. For example, in some scenarios, users use MAE as their evaluation metric to assess the imputation accuracy, while some other users with their applications prefer to use MSE to evaluate the final imputation results. From the perspective of helping our users get better results, we should add such a feature.

3. Your contribution

Will create a PR to finish it.