GroupLasso support FeaturePytrees

[BalzaniEdoardo]

GroupLasso works with array only so far. Extend the method to support FeaturePytrees. This may require changing the code for the proximal operator.

The mask needs to be a dictionary with keys that matches the FeaturePytree. Entries is a 1D vector of zeros and one, with shape (n_neurons, ), indicating if a feature needs to be penalized or not for each neuron.