Add delay loop and support forward in Gaussian backend

[Hugh-888]

Update details

1. Support non-adjacent BS gate

2. Support adding delay loop in Gaussian backend using circ.delay

2.1 Support multiple delay loops acting on single wire

Using circ.unfold_circuit to check the equivalent circuit after forward.

2.2 Support multiple delay loops acting on the corresponding wires

3. Able to generate cluster state using delay loop, such as EPR, GHZ state