Fix import of mirror qv circuit function

[cosenal]

An explicit import of the mirror qv circuit function was missing; adding it for coherence with the rest of the module.

This solves the issue reported by @Shivansh20128 while working on #2274

I ran this line

circuit= benchmarks.mirror_qv_circuits.generate_mirror_qv_circuit(num_qubits=10, depth=2)

But I got the attribute error:

AttributeError: module 'mitiq.benchmarks' has no attribute 'mirror_qv_circuits'.

Note

Absolute vs. relative, and explicit vs. implicit imports is somehow a recurrent source of confusion/issue for users in Mitiq (more so than in another Python packages it seems.) This is because we haven't done a great job at pre-importing modules in dunder init files, neither we have defined a dynamic loading rule of the modules. For an explanation of the issue, see #2471 (comment)

That being said, for this specific case, the fix is straightforward, it was just a miss of a previous PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.72%. Comparing base (45778e8) to head (18f1e14).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2570   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          92       92           
  Lines        4167     4168    +1     
=======================================
+ Hits         4114     4115    +1     
  Misses         53       53           

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