Implement Gramian based aggregators as such

[PierreQuinton]

This concerns:

• UPGRAD
• MGDA
• DualProj
• PCGrad
• IMTL-G
• CAGrad
• Nash-MTL
• Aligned-MTL

This is a strict improvement and is a first step towards Gramian based JD.

[ValerianRey]

Yes, for that we would need a third class hierarchy (the first is Aggregator, the second is _Weighting) that would be something like _GramianWeighting.

It would have the responsibility of giving weights (vector of shape [m]) from a Gramian (matrix of shape [m, m]).

I think I'd like to work on statefulness first, as it will also affect class hierarchies.

[ValerianRey]

I'm actually not sure about what to do for stateful aggregators, so we will probably do this first.