Adding a new argument to 'plot_state_qsphere'

[pragyakatyayan]

Motivation

I ran into this situation where only some states were visible while visualizing the qsphere few days back and asked help here. It was then that @aasfaw motivated me to do something like this. Many thanks to him. See the conversation here.

Summary

The previous show_state_labels argument is meant to show labels for only weighted states. When used with show_state_phases=True the weighted states displayed their phase values. The states with 0 weights were not given the labels.

The new 'optional' argument show_all_state_labels which is added with this PR shows all possible state labels when set to True. This Boolean variable which is false by default, aims at letting the end-user distinguish visually on the q-sphere between the states having zero weights and those having non-zero weights. The argument is kept optional to ensure that it is set to true only if the user wants to display all possible state labels for any particular circuit, the default view would show labels (and phases) for non-zero states only.
The new argument works well with show_state_phases=True and shows phase values for all possible state labels.
The test cases given below are performed for two circuits-one with 2 qubits and other with 3 qubits. Both the circuits were deliberately made to produce weights for half of the possible states, to notice the working of new argument for states with zero-weights. Snapshots show the default view and the new argument's output.

Details and comments

Case 1: Tested for two-qubit circuit (clutter possibility is low)

Default:

New Argument:

with show_state_phases=True

Default:

New Argument:

---

Case 2: Tested for three-qubit circuit (clutter possibility is medium-high)

Default:

New Argument:

with show_state_phases=True

Default:

New Argument:

P.S.: This is my first attempt with PR on Github.

[CLAassistant]

All committers have signed the CLA.