Enable channels in fidelity op

[lockwo]

Currently using channels (noise or reset, either way non-unitary) is not supported in the fidelity operation. Perhaps the fidelity could be modified or a "noisy fidelity" could be added which approximates it. This was discussed in the 8/19/2021 TFQ meeting.

[MichaelBroughton]

To anyone looking to implement this, It would entail making a new custom op, very similar to https://github.com/tensorflow/quantum/pull/554, but instead making use of our qsim trajectory simulator and requiring a repetitions parameter.