Add Qiskit CollectCliffordspass to UCC defaults

[Misty-W]

Fixes #60

Add Qiskit CollectCliffords pass to UCC defaults to reduce multi-qubit gate counts.

Per the Qiskit transpiler API doc, the CollectCliffords pass collects blocks of Clifford gates and replaces them with an N-qubit unitary operator from the Clifford group. More details on Clifford objects in Qiskit can be found at: https://docs.quantum.ibm.com/api/qiskit/qiskit.quantum_info.Clifford.

In this PR, the CollectCliffords pass is appended at the end of the add_local_passes method of the class UCCDefault1. The UCC benchmarks are re-run to reflect the resulting improvements in gate counts. Theoretically, the additional pass should incur a small compile time cost, but when compared to the version of the benchmark notebook on main, my local version showed a slight decrease in compile time, i.e. 1.9s on main (no additional pass) vs. 1.2s locally (with pass added). The numbers are close enough that I would expect in most cases the compile time cost to be acceptable, given the gate reduction improvements on multiple benchmarks.

[Misty-W]

Since this PR adds an unmodified Qiskit pass to UCCDefault1, we can go directly to acceptance criteria for being incorporated into UCCDefault1.

• Reduction in compiled 2 qubit gate count:
  Before:
  
  After:
  

• Key metric: increase in "gate reduction factor per second" -- as defined in the UCC benchmarks notebook:
  Before:
  
  After:
  

[jordandsullivan]

Lgtm. Do we know any Qiskit isn't using this pass in their default?

[Misty-W]

Lgtm. Do we know any Qiskit isn't using this pass in their default?

Thanks Jordan! I haven’t come across any indications as to why it’s not default, but would be good to be aware. Maybe the folks on the earlier thread about support for parametric gates would know.