Update analytic expression in variance test

[antalszava]

Context: A test case for a displaced squeezed circuit was added for calculating variances. However, the analytic expression provided was incorrect.

Description of the Change: Updates the analytic expression used for the displaced squeezed variance test.

Benefits: The tolerance value used can be decreased.

Possible Drawbacks: N/A

Related GitHub Issues: N/A

[codecov[bot]]

Codecov Report

Merging #436 into master will increase coverage by 0.09%. The diff coverage is 99.35%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
+ Coverage   97.84%   97.94%   +0.09%     
==========================================
  Files          58       69      +11     
  Lines        6784     7096     +312     
==========================================
+ Hits         6638     6950     +312     
  Misses        146      146              

Impacted Files	Coverage Δ
strawberryfields/apps/plot.py	100.00% <ø> (ø)
strawberryfields/apps/train/embed.py	100.00% <ø> (ø)
...awberryfields/backends/gaussianbackend/__init__.py	100.00% <ø> (ø)
strawberryfields/backends/gaussianbackend/ops.py	100.00% <ø> (+2.70%)	:arrow_up:
strawberryfields/circuitspecs/circuit_specs.py	100.00% <ø> (ø)
strawberryfields/circuitspecs/gaussian_unitary.py	100.00% <ø> (ø)
strawberryfields/circuitspecs/tensorflow.py	100.00% <ø> (ø)
strawberryfields/backends/fockbackend/ops.py	94.04% <94.11%> (+2.04%)	:arrow_up:
strawberryfields/ops.py	98.84% <96.29%> (-0.19%)	:arrow_down:
strawberryfields/backends/tfbackend/ops.py	96.69% <97.88%> (-0.17%)	:arrow_down:
... and 49 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e980b02...5d89f0a. Read the comment docs.

[antalszava]

• Was revisiting analytic expressions for https://github.com/XanaduAI/pennylane-sf/pull/42 and this turned out to be amiss. :slightly_smiling_face:
• Yes, the increased tolerance made it pass. :confused: If the parameters for displacement and squeezing are high and the Fock backend is used (with lower cutoff values), then the difference between the obtained result for the variance and the analytic value can be greater than the standard tolerance used here. However, it turned out this was not the case.