feat: allow for gather op with private/advice indices

[alexander-camuto]

• Allows for private / advice indices for indices in the gather op
• This unlocks models like sklearns' decision tree -- which we convert to a nice pytorch graph using sk2torch
• We add decision tree models from sklearn to the CI
• The dynamic gather op also allows us to implement ops like argmax and argmin which we also add here !

Resolves #99