Bug: nn.grad for floating variables

[KazukiYoshiyama-sony]

https://github.com/sony/nnabla/blob/v1.20.1/python/src/nnabla/grad.py#L71

This 0 should be 0 * nn.Variable(o.shape). Otherwise, grad(grad(concatenation([x, y, z]), x), x) does not work since [dy, dz] are floating variables.