Only update object's queue metadata if already in the queue

[albi3ro]

In multiple places across the codebase, we see the pattern:

try:
    qml.QueuingContext.update_info(obj, **kwargs)
except QueuingError:
      obj.queue()
     qml.QueuingContext.update_info(obj, **kwargs)
except NotImplementedError:
    pass

Currently, the role of object metadata is to indicate whether an operation should be ignored by the tape processing step. So most of the time, there is no reason to queue the object only to mark it as invisible.

This also is a lot of repeated code.

This PR adds a safe_update_queue and _safe_update_queue methods to qml.QueuingContext that only updates the metadata if the object is in the queue. It will never raise QueuingError's.

[codecov]

Codecov Report

Merging #2612 (881df0d) into master (5e411af) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2612      +/-   ##
==========================================
- Coverage   99.59%   99.58%   -0.02%     
==========================================
  Files         244      244              
  Lines       19670    19667       -3     
==========================================
- Hits        19590    19585       -5     
- Misses         80       82       +2     

Impacted Files	Coverage Δ
pennylane/measurements.py	99.57% <100.00%> (-0.01%)	⬇️
pennylane/operation.py	96.50% <100.00%> (-0.33%)	⬇️
pennylane/ops/qubit/hamiltonian.py	100.00% <100.00%> (ø)
pennylane/queuing.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5e411af...881df0d. Read the comment docs.

[Jaybsoni]

Looks good 👍🏼

[Jaybsoni]

Just CodeCov 👍🏼