Add MPS-based circuit

[SUSYUSTC]

• Add FiniteMPS class inherited from tensornetwork.FiniteMPS to fix the bug

• Add MPSCircuit with the following main functionalities:

  1. Initialization.
  2. Set truncation rules.
  3. Apply single, double qubit gate. In principle, MPS accepts only adjacent double qubit gates, but general double qubit gates can be achieved by N sequential SWAP gates (also implemented)
  4. Measurement and mid-measurement
  5. Compute the wavefunction
  6. Construction of MPS from a given wavefunction (with and without truncation)
  7. Copy and conjugate
  8. Projection (i.e. inner product) Computation <bra|ket>
  9. Expectation calculation

• Tests of most of the above functionalities based on

  1. Comparison between Circuit and MPSCircuit
  2. Comparison between MPS-based calculations and wavefunction-based calculations

The code should be backend-general, but only the numpy backend is tested.

[refraction-ray]

The MPS implementation seems good in general. I believe some minor changes are needed as I commented above. Specifically, we need further test these new functionalities on tensorflow and jax backend, especially with the interplay between them and jit or grad. And the PR should pass all tests in ./check_all.sh