Support qml.counts for circuits

[king-p3nguin]

Issue #, if available:
#195

Description of changes:

• qml.counts() is now supported.

Testing done:

Example:

import pennylane as qml
dev = qml.device("braket.local.qubit", wires=2, shots=100)

@qml.qnode(dev)
def test_circ():
    qml.PauliX(0)
    return qml.counts()

test_circ()

output:

tensor(Counter({'10': 100}), dtype=object, requires_grad=True)

Merge Checklist

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[yitchen-tim]

Hi @king-p3nguin, thanks for starting your contribution! Could you also add unit tests to this PR?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7b9f700) to head (3df7f89).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #267   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1211      1229   +18     
  Branches       295       304    +9     
=========================================
+ Hits          1211      1229   +18     

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[yitchen-tim]

Hi @king-p3nguin, the basic use case from the issue throws error

import pennylane as qml
dev = qml.device("braket.local.qubit", wires=2, shots=100)

@qml.qnode(dev)
def test_circ():
    qml.PauliZ(0)
    return qml.counts()

test_circ()

with error message DeviceError: Unsupported observable: <class 'NoneType'>

And another use case

@qml.qnode(dev)
def circuit():
    qml.X(wires=0)
    return qml.counts(op=qml.PauliZ(wires=[0]))

throws a different error NotImplementedError: Unsupported return type: ObservableReturnTypes.Counts

[yitchen-tim]

Also, could you add more tests to increase the coverage? thanks

[king-p3nguin]

@yitchen-tim Thank you for checking. The two basic use cases did not show errors in my environment. Can you ensure the code is executed within my fork's counts branch? (The added integrated tests have a similar circuit, and those tests are working fine on CI)

(As for unit testing, please give me some time as it seems to be quite complicated to create a mock)

[yitchen-tim]

@king-p3nguin Great, thanks, I did have some dependencies issues so it was not working correctly. The basic examples works for me now. After full test coverage, we should be good to go!

Also, per UnitaryHack guideline, it's good as long as PR is opened before EOD today and merged before 6/19. So we still have a week.

[yitchen-tim]

Can you add test case for unsupported return type rather than # pragma: no cover?

[king-p3nguin]

It seems that when an observable is defined, there are only four possible types (Expectation, Variance, Sample, and Counts), so I will just use the else statement here.

[yitchen-tim]

The else statement is prone to error. Pennylane supports few other return types, and the else statement doesn't really handle all these other result types.

To add coverage and test the unsupported return type for these lines

    else:
        raise NotImplementedError(f"Unsupported return type: {return_type}")

, you could mock the measurement as something like

measurement = Mock(
    return_type = Enum('ObservableReturnTypes', {'Foo': "foo"}).Foo,
    observable = qml.PauliZ()
)

If any function that is imcompatible with the mocked measurement, you can adjust the mock or patch the imcompatible function.

[king-p3nguin]

Thank you for the suggestion! I made the change as requested.

[king-p3nguin]

@yitchen-tim
Sorry for the delay. I added unit tests.

[yitchen-tim]

@king-p3nguin thanks, coverage is 100%, woot! Just one more change around testing unsupported result types.

[yitchen-tim]

Great work! Thanks again for the contribution @king-p3nguin