[ENH] ensure that model metadata/package class can generate instances

[fkiraly]

For testing the neural network models, we need a way to generate combinations of instances of the D2 class, and the model.

This should be the equivalent of create_test_instances_and_names in sktime and adjacent libraries.

First this should work for v1, then for v2.

What I would suggest is:

• adopt the design in [API] M layer design #1870
• simply add get_test_params
• in this case the individual instances will be results of calling the package class on a dataset

[fkiraly]

FYI @agobbifbk, @phoeenniixx, @PranavBhatP