Add Vendi Score and Memorization metrics for generative model evaluation

@jdipalma2 is already working on this.

Since Sep 9, 2025.

## 🚀 Feature

I would like to add the Vendi Score (https://arxiv.org/abs/2210.02410, https://arxiv.org/abs/2310.12952) and the L2-memorization percentage (https://arxiv.org/abs/2301.13188) for evaluating the performance of generative image models.

### Motivation

These are useful metrics for evaluating the holistic performance of generative models that go beyond the simple FID metric.

### Pitch

I would like to implement these metrics and contribute them to this library.

### Additional context

There are existing implementations of the Vendi Score (https://github.com/vertaix/Vendi-Score) and L2-memorization percentage (https://github.com/layer6ai-labs/dgm-eval) but they aren't incorporated into the torchmetrics API.