Refactor `NonlinearConjugateGradientOptimizer` #3216

Job	Run time
ubuntu-20.04-gcc-9 Release Python 3	17m 2s
ubuntu-20.04-clang-9 Release Python 3	16m 52s
macos-12-xcode-14.2 Release Python 3	19m 31s
macos-14-xcode-15.4 Release Python 3	11m 38s
windows-2022-msbuild Release Python 3	30m 22s
	1h 35m 25s

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