[WIP] Fix memory usage issues in BasisTranslator, DAGFixedPoint and Decompose transpiler passes, fixes issue #7485

[TheGupta2012]

Summary

Fix memory usage issues in BasisTranslator, DAGFixedPoint and Decompose transpiler passes, fixes issue #7485

Details and comments

Decompose

• The original _should_decompose method tried to make two variables strings_list and gate_type_list repeatedly during the call of the method.
• This should not be the case as the lists only depend on the gates_to_decompose attribute of the pass, which is invariant during the call of the run method
• New attributes are introduced for the strings_list and the gate_type_list lists.
• Although this may not have much effect on the memory usage of the pass, this should have some improvement in the run time of the pass.

BasisTranslator - to do

BarrierBeforeFinalMeasurements

• There is a dependency of MergeAdjacentBarriers in the above pass.
• The variable current_barrier_nodes here was used only for membership checking
• A set would serve as a more optimal data structure here
• Note : Memory leakage is yet to be detected

DAGFixedPoint

• There was a usage of deepcopy, for saving a copy of the dag, in the property set of the circuit.
• This may have been a point of memory leakage and thus an explicit delete was introduced before the re-assignment.

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[coveralls]

Pull Request Test Coverage Report for Build 2228333600

• 15 of 15 (100.0%) changed or added relevant lines in 3 files are covered.
• 22 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.02%) to 84.264%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/circuit/controlflow/control_flow.py	1	83.33%
qiskit/circuit/controlflow/while_loop.py	1	97.96%
qiskit/transpiler/passes/optimization/optimize_1q_commutation.py	1	99.09%
qiskit/extensions/unitary.py	4	96.12%
qiskit/circuit/controlflow/if_else.py	7	96.77%
qiskit/transpiler/passes/basis/unroller.py	8	85.96%

Totals
Change from base Build 2208196800:	0.02%
Covered Lines:	54180
Relevant Lines:	64298

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💛 - Coveralls

[ewinston]

I tested 08a2b0d and didn't appear to see a significant change in the memory usage.