knn_filter - #357

>>> import torch, pypose as pp
>>> points = torch.tensor([[0.,  0.,  0.],
...                        [1.,  0.,  0.],
...                        [0.,  1.,  0.],
...                        [0.,  1.,  1.],
...                        [10., 1.,  1.],
...                        [10., 1., 10.]])
>>> pp.knn_filter(points, k=2, radius=5)
tensor([[0.3333, 0.3333, 0.0000],
        [0.3333, 0.3333, 0.0000],
        [0.0000, 0.6667, 0.3333],
        [0.0000, 0.6667, 0.3333]])