Update the developers plugin guide to use QubitDevice

[josh146]

Context: The new QubitDevice makes plugin development significantly easier. The existing Device API should be considered deprecated, and will eventually be removed from Device.

Description of the Change:

• Updates the plugin guide to focus on the new plugin API

• Cleans up some docstrings in QubitDevice

Benefits: n/a

Possible Drawbacks:

• Some plugins currently still use Device; they will need to be ported to QubitDevice.

• A CVDevice equivalent is still TODO.

Related GitHub Issues: n/a

[codecov]

Codecov Report

Merging #483 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #483   +/-   ##
======================================
  Coverage    99.1%   99.1%           
======================================
  Files          51      51           
  Lines        3910    3910           
======================================
  Hits         3875    3875           
  Misses         35      35

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[mariaschuld]

Awesome Josh, I haven't had much to do with plugin development but now I feel I could do it myself!

Two general things you may want to improve:

1. The "Device execution" section is still rather dense. Maybe use a few more sentences/examples to explain, since people also use different technical terms sometimes?
2. It is a bit confusing that there are CV/Qubit devices, then the QubitDevice is used throughout the explanation, and at the end there is a CVOperation. I think it's fine to focus on QubitDevice, but it should be clear how CV is different from that?

[josh146]

It is a bit confusing that there are CV/Qubit devices, then the QubitDevice is used throughout the explanation, and at the end there is a CVOperation. I think it's fine to focus on QubitDevice, but it should be clear how CV is different from that?

This was (unfortunately) deliberate 🙁 My thinking was:

• Almost all external plugin developers will be working with the qubit model

• There is no CVDevice yet; it is planned, but won't make it to this release.

What do you think is the least confusing approach? Once the CVDevice is ready, my plan was to create a CV device page. But it does raise questions in the interim.

Options are:

• Add a note to the page about CV (but not really sure what to say, since CVDevice doesn't exist yet)

• Remove the CV operation/observable stuff for now (but could still be useful for internal developers?)

[antalszava]

Left one comment about adding Identity to the set of observables

[codecov]

Codecov Report

Merging #483 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #483   +/-   ##
=======================================
  Coverage   99.05%   99.05%           
=======================================
  Files          51       51           
  Lines        3923     3923           
=======================================
  Hits         3886     3886           
  Misses         37       37

Impacted Files	Coverage Δ
pennylane/beta/__init__.py	100% <0%> (ø)	⬆️
pennylane/beta/plugins/__init__.py	100% <0%> (ø)	⬆️
pennylane/beta/plugins/expt_tensornet.py	97.68% <0%> (ø)	⬆️
pennylane/beta/plugins/expt_tensornet_tf.py	92.03% <0%> (ø)	⬆️
pennylane/qnodes/device_jacobian.py	90.9% <0%> (ø)	⬆️
pennylane/interfaces/tf.py	93.02% <0%> (ø)	⬆️
pennylane/qnodes/decorator.py	100% <0%> (ø)	⬆️
pennylane/qnodes/jacobian.py	100% <0%> (ø)	⬆️
pennylane/_device.py	99.27% <0%> (ø)	⬆️

---

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[mariaschuld]

Better: will be available soon? I learnt to not promise things if not absolutely necessary :)

[mariaschuld]

Thanks for making the changes, approved from my side.