Qiskit
Introduce fractional gates
Closes #1526 (and will likely close #1471 @lerongil).
FYI @nbronn here's the draft of the fractional gates page.
Check out this pull request on 
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In addition to the comments I added from ReviewNB, we also need a section on when NOT to use fractional gates. In particular, we currently do not support using fractional gates with dynamic circuits or with any error mitigation features of the Qiskit Runtime. Furthermore, the Qiskit transpiler has limited capability to use rzz in its optimization passes. So, this is mostly relevant for users hand-crafting circuits and using the Sampler.
@abbycross Can you also mention that the "error" value of rx (rzz) gate reported in Target is a copy of non-fractional counterpart, i.e. sx (cz) gate? This is worth noting because user may want to choose optimal decomposition based on net error, but this strategy doesn't work currently because the actual error of fractional gates are not reported.
Since gate time of fractional/non-fractional gates are the same, we can expect their error values are also almost comparable when dominant source of error is relaxation, but they could differ because of the difference in calibration protocol.
@abbycross Can you also mention that the "error" value of rx (rzz) gate reported in
Targetis a copy of non-fractional counterpart, i.e. sx (cz) gate? This is worth noting because user may want to choose optimal decomposition based on net error, but this strategy doesn't work currently because the actual error of fractional gates are not reported.Since gate time of fractional/non-fractional gates are the same, we can expect their error values are also almost comparable when dominant source of error is relaxation, but they could differ because of the difference in calibration protocol.
Just added this as a note in the first section of the page. @blakejohnson and @javabster I believe this page should be ready to go now that fractional gates are live.
