Reset Transpiler When Setting Backends

[csookim]

Changing the backend does not reset the transpiler, which can cause potential errors.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.72%. Comparing base (faa5a7c) to head (d4cbe05).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1526   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          80       80           
  Lines       11783    11784    +1     
=======================================
+ Hits        11751    11752    +1     
  Misses         32       32           

Flag	Coverage Δ
unittests	99.72% <100.00%> (+<0.01%)	⬆️

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