bug fix in VoxelGrid code

[yindaheng98]

1. some typos cause "variables not found": samples_result/samples_reuslt, dirs/viewdirs
2. gradient and backward cannot reach two nn.Parameter sh_fields and density_fields since torch.cat((self.sh_fields, self.density_fields), dim=3). The proper way is to use them separately
3. the trilinear_interpolation seems wrong, fixed