Allow clip_by_value to get scalar values as threshold

[TakuyaYashima]

Currently, clip_by_value's input is either Variable or NdArray, even for threshold values(min and max). It seems sometimes we just want to pass the scalar value as threshold, so I modified it to take the scalar input as well.

Now it works like;

x = nn.Variable((...))
y = F.clip_by_value(x, 0., 1.)
y.forward() # works