API change

[viking-sudo-rm]

Should change API from classmethod empty() and method update() to two methods reset()and update().

This would make get_num_actions() a property of an instance, not a class. This makes it easier to change the number of actions for multipush/multipop stacks.

[viking-sudo-rm]

Should be able to do this in a way that doesn't break old API. In other words, we define

@classmethod
def empty(cls, batch_size, stack_dim, device):
    stack = cls(stack_dim)
    stack.reset(batch_size, device)
    return stack

[viking-sudo-rm]

Fixed in 95446f2.