assignment: add ISWAP functionality and unit tests

[luotianwu]

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Context:

Description of the Change:
Add ISWAP functionality in BasicGateFunctors.hpp
Add ISWAP unit tests in :
StateVectorKokkos::applyMatrix/Param-Operation
StateVectorKokkos::applyISWAP
Passed updated tests in lightning_kokkos_gates_test_runner

Benefits:
New ISWAP feature available and can run on multiple HPC platforms.

Possible Drawbacks:
Python layer testing for this gate was not addressed.
Performance test for this gate was not conducted.

Related GitHub Issues:

Fixes #740.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.20%. Comparing base (e24ba31) to head (79f1fd5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #749      +/-   ##
==========================================
- Coverage   94.57%   91.20%   -3.38%     
==========================================
  Files         122       88      -34     
  Lines       17781    10627    -7154     
==========================================
- Hits        16816     9692    -7124     
+ Misses        965      935      -30     

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[vincentmr]

Thanks for initiating this @luotianwu. Could you compare the performance of the ISWAP gate on lightning.kokkos between the SERIAL and OpenMP backends and explain how the ISWAP C++ Kokkos kernel is executed concurrently. Sweeping over target qubits you should see a hardware dependent pattern, can you explain where it comes from?

[luotianwu]

The ISWAP kernel is launched by Kokkos::parallel_for in applyNC2Functor. The kernel will be executed in parallel over range 0-exp(num_qubits-2)-1. The kernel execution for different indices in the range can be assigned to multiple processing units. The state vector array has size exp(num_qubits), by operating two of the qubits, the state vector indices are grouped by 4 and thus needs to loop over exp(num_qubits-2) groups.
The hardware dependent pattern is passed by template parameter ExecutionSpace. The value passed for it is Kokkos::DefaultExecutionSpace. I think it get value from CMAKE configuration -DKokkos_ENABLE_OPENMP=ON, -DKokkos_ENABLE_CUDA=ON, -DKokkos_ENABLE_SERIAL=ON etc.

I tried to build with -DKokkos_ENABLE_OPENMP=ON and -DKokkos_ENABLE_CUDA=OFF. To see the performance, I added some time collection code in the kernel and also printed out the backend by KokkosExecSpace::name(). But somehow it always show CUDA as the backend. Unfortunately I didn't have more time to figure it out.