rightapplyCompMatr distributed input validation

[otbrown]

Spun out from discussion on #699. When running on more than one process the rightapplyCompMatr validation tests fail as no exception is thrown in the "target amps fit in node" test, as below. (I've slightly expanded the CAPTURE in the below output!)

mpirun -n 2 ./tests/tests rightapplyCompMatr

QuEST execution environment:
  precision:       2
  multithreaded:   1
  distributed:     1
  GPU-accelerated: 0
  GPU-sharing ok:  0
  cuQuantum:       0
  num nodes:       2

Testing configuration:
  test all deployments:  0
  num qubits in qureg:   6
  max num qubit perms:   10
  max num superop targs: 4
  num mixed-deploy reps: 10

Tested Qureg deployments:
  CPU + OMP + MPI

Filters: "rightapplyCompMatr"
Randomness seeded to: 4228350052

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tests is a Catch2 v3.8.0 host application.
Run with -? for options

-------------------------------------------------------------------------------
rightapplyCompMatr
  validation
  targeted amps fit in node
-------------------------------------------------------------------------------
/home/otbrown/quantum/QuEST/tests/unit/operations.cpp:1043
...............................................................................

/home/otbrown/quantum/QuEST/tests/unit/operations.cpp:1085: FAILED:
  REQUIRE_THROWS_WITH( apiFunc(), ContainsSubstring("cannot simultaneously store") && ContainsSubstring("remote amplitudes") )
because no exception was thrown where one was expected:
  minNumCtrls := 0
  numNewTargs := 6
  numQubits - minNumCtrls := 6
  ctrls := {  }
  targs := { 0, 1, 2, 3, 4, 5 }
  matr.numQubits := 6
  qureg.numQubits := 6
  qureg.logNumAmpsPerNode := 11

===============================================================================
test cases:  1 |  0 passed | 1 failed
assertions: 61 | 60 passed | 1 failed

--------------------------------------------------------------------------
Primary job  terminated normally, but 1 process returned
a non-zero exit code. Per user-direction, the job has been aborted.
--------------------------------------------------------------------------
--------------------------------------------------------------------------
mpirun detected that one or more processes exited with non-zero status, thus causing
the job to be terminated. The first process to do so was:

  Process name: [[50347,1],0]
  Exit code:    42
--------------------------------------------------------------------------

I assume this test is attempting to trigger an exception from validate_mixedAmpsFitInNode. The problem is that asserts that qureg.numAmpsPerNode >= numTargAmps, and it always will be here, as qureg.numAmpsPerNode accounts for the fact that the qureg is a density matrix, while numTargAmps is calculated as 2^numTargets.

I would have said then that this just needed a slight change in the logic to account for density matrices, except there's a comment directly above it which says that isn't necessary so I'm throwing it back to you @TysonRayJones 😅

[TysonRayJones]

The problem is that asserts that qureg.numAmpsPerNode >= numTargAmps, and it always will be here, as qureg.numAmpsPerNode accounts for the fact that the qureg is a density matrix, while numTargAmps is calculated as 2^numTargets

That logic (and the aforementioned comment) is actually correct! Indeed being a density matrix does not change the "fits into node" logic. We can think of an N-qubit density matrix as a 2N-qubit unnormalised statevector; qureg.numAmpsPerNode will correspond correctly. Functions leftapplyCompMatr() and rightapplyCompMatr() merely differ in the targeted qubits of this unnormalised statevector, but their validation logic is identical.

The error is that the unit test incorrectly expects the assertion to fail. The assertion succeeds because my previous switch to use a density matrix has increased numAmpsPerNode as you observe. I'll change the conditions for when the failure is expected 🙏

[otbrown]

Excellent, thanks Tyson! I've merged your patch to devel, and will close this.