Note: t-SNE and UMAP, nonlinear dimensionality reduction for visualization

Implement:

def gaussian_affinities(input: np.ndarray, sigma: float) -> np.ndarray:
    """Returns shape (n_samples, n_samples): a row-normalized similarity
    distribution, p[i, i] = 0."""

• Reuse 01-knn's pairwise_distances.