CircuitSampler drops is_measurement status when converting StateFns

[dongreenberg]

If you look at the code below, you'll find that the measurements have been replaced with StateFns, making the ComposedOps invalid and unable to eval. The same is true for statevector mode.

Code:

import qiskit.aqua.operators as of
from qiskit import BasicAer
data = of.ListOp([of.Plus, of.Zero])
qk = ~data @ data
print(qk)
sampled_qk = of.CircuitSampler(backend=BasicAer.get_backend('qasm_simulator')).convert(qk)
print(sampled_qk)

I believe the issue is that on lines 309, 311, and 314 of the CircuitSampler the resulting StateFn should be given the measurement status of op_c.is_measurement, but I'm not 100% positive.

[molar-volume]

After setting is_measurement=op_c.is_measurement, it is possible to eval, and for that particular example we get

print(sampled_qk.eval())  # [[1.0, 0.7022564079451321], [0.6803606580336637, 1.0]]

@jlapeyre If you did not start already, I can make the PR for this issue and #1338 .
But if you think that problem is more tricky, I can leave it up to you.

[jlapeyre]

@molar-volume , I won't start it myself. I'll look at your PR

[dongreenberg]

Thanks gang! When's the next release?