Add return_set_frac and return_set_n_samples to modadd

[danbraunai-apollo]

We can add parameters for return_set_frac and return_set_n_sampels to modular arithmetic. Pseudocode will be something like:

This could be handled inside of create_modular_arithmetic_dataset by sampling from raw_dataset.

Once implemented, use a small sample of the dataset in all of the modular addition tests where it makes sense (which is probably all of them?). And uncomment test_build_graph.test_modular_arithmetic_rotate_final_layer_invariance.