[Feature Request] Support plugin for loss (in pytorch backend)

[ChiahsinChu]

Summary

Currently, the loss function module is chosen by deepmd.pt.train.training.get_loss based on the loss type keyword. This means that the user cannot implement a custom loss function module in a plugin, without changing the code in the deepmd-kit package. I wonder whether it is possible to have a hook for loss function module, as what has been done for the descirptors, etc.

Detailed Description

See above.

Further Information, Files, and Links

No response