Fix RemoteEngine.run_async kwargs bug, and add test

[josh146]

Context:

The RemoteEngine.run_async() method had the erroneous line

kwargs = kwargs.update(self._backend_options)

The dict.update method is an in-place method, and returns None, resulting in an incorrectly compiled blackbird program.

Description of the Change:

• The line above has been modified to remove the assignment.

• A tests has been added to catch this behaviour in future.

• The logic in determining the run options has been moved to RemoteEngine.run_async, as it makes more logical sense here than as part of the Connection object.

• I noticed that SF was not providing a default value for the number of shots in the remote engine. Local engine has shots=1 by default, but this is because the fock and tf backends don't support multishot sampling yet, so isn't as applicable to the remote engine. The RemoteEngine.run_async method now raises an error if number of shots is not provided.

Benefits:

• Fixes a bug

• More robust remote engine tests

Possible Drawbacks: n/a

Related GitHub Issues: n/a

[codecov[bot]]

Codecov Report

Merging #343 into master will decrease coverage by 0.03%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
- Coverage    97.7%   97.67%   -0.04%     
==========================================
  Files          51       51              
  Lines        6354     6355       +1     
==========================================
- Hits         6208     6207       -1     
- Misses        146      148       +2

Impacted Files	Coverage Δ
strawberryfields/api/connection.py	97.61% <100%> (-0.09%)	:arrow_down:
strawberryfields/engine.py	95.45% <100%> (+0.12%)	:arrow_up:
strawberryfields/backends/tfbackend/ops.py	95.74% <0%> (-0.35%)	:arrow_down:

---

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 0c2520e...5353e3a. Read the comment docs.