pydantic inherentance

[roussel-ryan]

All Xopt generators are subclasses of the pydantic (v2) BaseModel class (https://docs.pydantic.dev/latest/). This enables generator serialization that may or may not be useful. Serialization of arbitrary types is not supported by native pydantic types, but pydantic is flexible enough to support custom serialization. For example, we have serialized complex objects such as torch modules in the past.