chore: add the `mlp_engine` option

[njzjz]

I am going to use DP-GEN to develop models trained by other MLP software. This may or may not be merged into the main branch, but I think a general mlp_engine option can be added anyway.

Summary by CodeRabbit

• New Features

  • Introduced handling for multiple ML potential engines with specialized training argument functions.

• Improvements

  • Enhanced training initialization by splitting into common and engine-specific functions.
  • Improved error handling for unsupported ML potential engines.

• Bug Fixes

  • Corrected logic to differentiate between dp and other engine values during training and model initialization.