Move Coupler to cirq_gooogle.ops

[dstrain115]

• Noticed when adding code that there were circular imports due to Couplers api codes needing code from devices.
• Moved this to ops directory instead, which avoids the circular import.
• Note that, although GridQubit is in cirq.devices, LineQubit and NamedQubit are in cirq.ops, so this also seems to be a good place to put this class.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.82%. Comparing base (978b5e3) to head (47c08da).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6697 +/- ## ======================================= Coverage 97.82% 97.82% ======================================= Files 1074 1074 Lines 92191 92187 -4 ======================================= - Hits 90189 90186 -3 + Misses 2002 2001 -1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.