Enable qpu.backend for IonQ backends

[splch]

use the new backend field:

dev = qml.device(
    'ionq.qpu',
    backend='aria-1',
    api_key=api_key,
    wires=2,
)

to call qpu.aria-1

[lillian542]

Thank you for your contribution to the plugin, this looks great! 🎉

I would like to request one change in the implementation - I’m hesitant to add hardware-specific subclasses for different machines when there is no functional difference that needs to be supported. The call signature

qml.device(“ionq.qpu”, backend=“aria-1”, api_key=api_key, wires=2)

works well, and it won’t require a new version of the plugin to be released if IonQ adds, removes, or changes the names of any of the devices. This is generally also what we’ve done with other plugins - for examples, see the Honeywell or Rigetti plugin documentation.

Other than that, there are just a couple of small things that need to be added for tests and documentation :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a967df1) 95.73% compared to head (2f491bd) 93.79%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   95.73%   93.79%   -1.95%     
==========================================
  Files           5        5              
  Lines         305      306       +1     
==========================================
- Hits          292      287       -5     
- Misses         13       19       +6     

Files	Coverage Δ
pennylane_ionq/device.py	93.04% <100.00%> (-2.58%)	⬇️

... and 1 file with indirect coverage changes

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[lillian542]

Thanks! Can you also run black -l 100 pennylane_ionq/ to update whatever the formatting check is bothered about?

The CodeCov "reduced coverage" looks to me like it's an artifact of reformatting and not a change in how much of the plugin functionality is covered by the tests.

[lillian542]

Looks good, thanks!