Output naming

The name and format of the outputs in metatrain are based on those of the metatensor.torch.atomistichttps://docs.metatensor.org/latest/atomistic/outputs/index.html package. An immediate example is given by the energy output.

Any additional outputs present within the library are denoted by the mtt:: prefix. For example, some models can output their last-layer features, which are named as mtt::aux::{target}_last_layer_features, where aux denotes an auxiliary output.

Outputs that are specific to a particular model should be named as mtt::<model_name>::<output_name>.