Migrate to Pytorch Lightning 2.0.0

Some APIs are changed in Pytorch Lightning 2.0.0. In training:

• validation_epoch_end is removed. Recommend using on_validation_epoch_end. That requires the user to save the validation outputs
• Trainer does not have gpus options. Instead, one can use devices.
• the optimizer_step API is changed. https://lightning.ai/docs/pytorch/stable/common/lightning_module.html#optimizer-step

There may be other changes that affect us.