Allow qml.cond to support passing operations

[trbromley]

Feature details

The qml.cond function is used as part of the mid-circuit meausurement functionality in PennyLane. Currently, the signature is qml.cond(condition, true_fn, false_fn), where true_fn and false_fn are quantum functions.

We would like qml.cond to accept operations in addition to quantum functions. The quantum function style can be useful in some cases, but having to then call qml.cond can feel a little counter-intuitive - instead the user might just want to pass the operation directly.

Implementation

We would like it to be possible to do:

qml.cond(m_0, qml.RY(par, wires=0), qml.RX(par, wires=0))

• The qml.RY and qml.RX are not directly queued, just contained within the qml.cond conditional
• Update true_fn and false_fn argument names in the signature of qml.cond to if_true and if_false, respectively (since _fn no longer always applies)
• The if_false operation remains optional
• If if_false is passed, it must match the type of if_true, i.e., function-function or operation-operation.
• Update docstrings of qml.cond. Note that the example itself doesn't need to be updated.

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

Similar functionality is supported in qml.adjoint.

[trbromley]

Closing this as we have decided that it is not such a good idea - it would be introducing a UI that is not compatible with cond-like functions in other libraries.