Rename `qml.Interferometer` to `qml.InterferometerUnitary`

Feature details

As per https://github.com/PennyLaneAI/pennylane/issues/1256, we’re changing qml.Interferometer, as it collides with a the template qml.templates.Interferometer.

A short summary based on the (updated) comments in the linked issue:

In this PR we need to rename the class, and then search for lines in the code where Interferometer is translated to an implementation.

Implementation

1. Check the default.gaussian device, and change the operation name there. More precisely, at the moment when default.gaussian encounters a gate called Interferometer, it expects it to have a param that represents the unitary which defines the interferometer’s matrix. This logic should now only be performed when it sees InterferometerUnitary. When it sees Interferometer it should not find a translation in the list, and will then decompose this operation into the gates defined in template.Interferometer. In other words, in future InterferometerUnitary is an operation that is natively supported by devices, while templates.Interferometer is an operation that is only defined by its decomposition.

1. The representation_revolver would also require a change such that circuit drawing is updated.

See details on the possible changes to be made:

• https://github.com/PennyLaneAI/pennylane/issues/1256#issuecomment-845350642

How important would you say this feature is?

2: Somewhat important. Needed this quarter.

Additional information

We’d like to have this in for v0.19.0.

[antalszava]

Hi @AkashNarayanan, that’s great to hear! 🙂 Feel free to ask any questions and open a work in progress PR.

[antalszava]

Closing with #1714 merged.