Fix for using `TDMProgram` with `timebins % concurrent_modes != 0`

[thisac]

Ready for review, but since it relies on changes in the td3-tdmprogram branch, it's best to wait till that one is merged before merging this PR. Thus the do not merge label.

Context: The TDMProgram will produce wrong results if the number of timebins is not an integer multiple of the number of concurrent modes. This is because the tdmprogram.unroll() function simpy multiplies the unique circuit with the number of shots, not taking into account which qumode has been measured most recently.

Description of the Change:

• The way multiple shots are calculated is changed. It now calculates each shot separately instead of stitching together several identical circuits at the end.

• The way the measured mode order is calculated is improved (and now works with the new shots calculation).

• TDMProgram is tidied up slightly by changing a few attributes to private and adding corresponding properties to access them.

Benefits: The following now works properly:

theta = [0] * 3
r = 3
shots = 2

prog = sf.TDMProgram(N=2)
with prog.context(theta) as (p, q):
    ops.Xgate(50) | q[1]
    ops.MeasureHomodyne(p[0]) | q[0]
eng = sf.Engine("gaussian")
res = eng.run(prog, shots=shots)
samples = res.samples

print(samples)

outputting

[[[ 0.5660403  49.47315214 49.76274829]]

 [[48.94508121 49.77226491 51.71599116]]]

Possible Drawbacks:

Related GitHub Issues: fixes #607

[codecov[bot]]

Codecov Report

Merging #611 (12b1f14) into master (ec08408) will increase coverage by 0.00%. The diff coverage is 98.59%.

@@           Coverage Diff           @@
##           master     #611   +/-   ##
=======================================
  Coverage   98.56%   98.56%           
=======================================
  Files          77       77           
  Lines        8893     8904   +11     
=======================================
+ Hits         8765     8776   +11     
  Misses        128      128           

Impacted Files	Coverage Δ
strawberryfields/program.py	100.00% <ø> (ø)
strawberryfields/tdm/tdmprogram.py	98.24% <98.38%> (+0.08%)	:arrow_up:
strawberryfields/api/devicespec.py	95.31% <100.00%> (+0.07%)	:arrow_up:
strawberryfields/engine.py	96.49% <100.00%> (ø)
strawberryfields/io.py	96.90% <100.00%> (ø)

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[fab1an-q]

Ah, I see. Very clever, thanks!

On Thu., Jul. 22, 2021, 18:07 Theodor, @.***> wrote:

@.**** commented on this pull request.

In strawberryfields/tdm/tdmprogram.py https://github.com/XanaduAI/strawberryfields/pull/611#discussion_r674952351 :

• @property
• def concurr_modes(self):
• """The number of concurrent modes in the program."""
• return self._concurr_modes

This is just Pythons way of saying that self._concurr_modes is private (and shouldn't be changed manually outside of the class). By having a property, the value can be retrieved but not set (you'd need to add a "setter" method as well to do that, which could involve extra validation etc.).

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