Measurement fixes and improvements

[stavros11]

This PR implements various improvements in measurements, particularly:

• Eliminates the circuit.measurement_gate and adds all measurements (with and without collapse) in the gate queue.
• Makes possible to reuse (add more gates to) qubits that are measured. If the collapse=True option was not used when measuring a qubit that is reused then it is enabled automatically, as I am not aware of any other physical way that a qubit can be reused without collapsing it.
• Fixes an issue with result = c.add(gates.M(0, 1, collapse=True)) which was out of sync with previous versions and the docs (thanks @igres26).
  Now the result is a MeasurementResult object which has result.samples(), result.frequencies() and also result.symbols which is a list of sympy.Symbols that can be used to condition later gates on the measurement outcomes.
• The result object returned by c.add is now available for non-collapse measurements too.
  For example

from qibo import gates
from qibo.models import Circuit

c = Circuit(2)
c.add(gates.H(0))
c.add(gates.H(1))
m0 = c.add(gates.M(0))
m1 = c.add(gates.M(1))
r = c()

then

• r is a CircuitResult object that contains the final state vector and all measurements.
• m0 is a MeasurementResult object that contains the measurements of qubit 0.
• m1 is a MeasurementResult object that contains the measurements of qubit 1.

This is complete in terms of developments but I am labeling it as "do not merge" as I would like to make sure it plays well with qibolab before merging.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (3a92d4f) compared to base (513d302).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #647   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           88        88           
  Lines        11663     11702   +39     
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+ Hits         11663     11702   +39     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/noise.py	100.00% <ø> (ø)
src/qibo/tests/test_gates_abstract.py	100.00% <ø> (ø)
src/qibo/backends/numpy.py	100.00% <100.00%> (ø)
src/qibo/gates/measurements.py	100.00% <100.00%> (ø)
src/qibo/gates/special.py	100.00% <100.00%> (ø)
src/qibo/models/circuit.py	100.00% <100.00%> (ø)
src/qibo/models/distcircuit.py	100.00% <100.00%> (ø)
src/qibo/states.py	100.00% <100.00%> (ø)
src/qibo/tests/test_dill.py	100.00% <100.00%> (ø)
src/qibo/tests/test_measurements.py	100.00% <100.00%> (ø)
... and 7 more

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[andrea-pasquale]

Thanks for implementing this @stavros11.
Everything looks good to me. Good job!

[wilkensJ]

The unitary() function of a circuit object does not work anymore if a measurement is added.

from qibo import gates, models
circuit = models.Circuit(1)
circuit.add(gates.X(0))
print(circuit.unitary())
# array([[0.+0.j, 1.+0.j],
#        [1.+0.j, 0.+0.j]])
circuit.add(gates.M(0))
print(circuit.unitary())

raises

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nfs/users/jadwiga.wilkens/qibo/src/qibo/models/circuit.py", line 902, in unitary
    return fgate.asmatrix(backend)
  File "/nfs/users/jadwiga.wilkens/qibo/src/qibo/gates/special.py", line 102, in asmatrix
    return backend.asmatrix_fused(self)
  File "/nfs/users/jadwiga.wilkens/qibo/src/qibo/backends/numpy.py", line 117, in asmatrix_fused
    gmatrix = gate.asmatrix(self)
  File "/nfs/users/jadwiga.wilkens/qibo/src/qibo/gates/measurements.py", line 140, in asmatrix
    raise_error(
  File "/nfs/users/jadwiga.wilkens/qibo/src/qibo/config.py", line 46, in raise_error
    raise exception(message)
NotImplementedError: Measurement gates do not have matrix representation.

[stavros11]

The unitary() function of a circuit object does not work anymore if a measurement is added.

Thanks for reporting this. It was fixed and tested in the latest push.