Fix nested parallel performance #1949
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doichanj
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Changelog: Bugfix
hhorii
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Summary
OpenMP nested parallel simulation for parallel experiments + parallel state update was very slow when using gate fusion.
This occurs when using parameter binding for 15 or more number of qubits.
Details and comments
Inside gate fusion, unitary simulator is called to calculate fused matrix and uses omp region frequently that waste thread resource for nested parallelism. This fix removes parallel regions for gate fusion.