Squin identity sites should be an argument instead of an attribute

[david-pl]

When you want to "build up" an operator that acts on multiple qubits, you'll often find yourself in a situation where you'll have to write

$$ X_i = \mathbb{1} \otimes \mathbb{1} \otimes ... \otimes X \otimes \mathbb{1} ... $$

in order to "embed" a single qubit operator on a larger Hilbert space.

In squin, you'd do something like

x = squin.op.x()
id_lhs = squin.op.identity(sites=i)
id_rhs = squin.op.identity(sites=n - i - 1)
x_i = squin.op.kron(id_lhs, squin.op.kron(x, id_rhs))

Unfortunately, you can't use this approach unless n and i are actual constant values. That means you can neither define a short-hand embed(n: int, idx: int, op: Op) (which we'd probably want as part of stdlib) nor can you use the above in a for loop. Also, you can't define an operator for different n.

To solve this, we'll have to make sites an argument rather than an attribute. The obvious downside is that a couple of different analysis will have a more difficult implementation, since you won't always be able to infer the number of sites from the identity anymore.