`Synthesizer::new`を非asyncにする #1927

Job	Run time
config	3s
deploy_model	9s
build_and_deploy (windows-2019, directml, x86_64-pc-windows-msvc, windows-x64-directml, directml,...	9m 2s
build_and_deploy (ubuntu-20.04, x86_64-unknown-linux-gnu, linux-x64-gpu, cuda, true)	4m 1s
build_and_deploy (macos-11, aarch64-apple-darwin, osx-arm64-cpu, cpu, false)	5m 13s
build_java_package	0s
build_xcframework	0s
download_test	0s
	18m 28s

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