Update QVM demo notebooks to use willow_pink

[pavoljuhas]

The following quantum virtual machine notebooks were updated to use
the new willow_pink processor by default:

• quantum_virtual_machine.ipynb
• qvm_basic_example.ipynb
• qvm_builder_code.ipynb
• qvm_stabilizer_example.ipynb

Small fixups in other reviewed notebooks.

Related to b/395705720

[pavoljuhas]

NB: can be also reviewed per-commit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (b1ff3ae) to head (40fcb1d).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7428      +/-   ##
==========================================
- Coverage   98.69%   98.69%   -0.01%     
==========================================
  Files        1112     1112              
  Lines       97993    97993              
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- Hits        96714    96711       -3     
- Misses       1279     1282       +3     

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[eliottrosenberg]

Thanks, Pavol! General comment for all of these notebooks: let's always make "willow_pink" the default and maybe not even list the others since they are very old and people probably shouldn't be using them.

[eliottrosenberg]

Let's make "willow_pink" the default and maybe not even mention the others since they are very old and don't represent current performance.

[pavoljuhas]

Done in 832175f.

I have reviewed other notebooks for their processor choice. The ones that run QVM simulations now all default to "willow_pink". I have left the old default processors in qubit_picking.ipynb where the low noise performance is not critical for the demonstrated function and the heatmap plots would become too dense for willow. I have also left the old default "weber" processor in virtual_engine_interface.ipynb which primarily demos interaction with Quantum Engine (processor choice, time reservation) and uses noise-free simulated processors.

I feel removing old processors might degrade some of the notebooks, for example, the virtual_engine_interface would end up having a choice of one processor instead of three.

[eliottrosenberg]

Slight preference to remove all of the references to Rainbow and Weber but it's okay if you don't want to.