Fix bug in AVX512 f16-gemm #648

Job	Run time
build-and-test / bazel-linux-aarch64-clang18	1m 30s
build-and-test / bazel-linux-x86_64-clang-18	2m 34s
build-and-test / bazel-linux-x86_64-gcc-9	1m 8s
build-and-test / cmake-android (arm64)	1m 36s
build-and-test / cmake-android (armv7)	1m 45s
build-and-test / cmake-android (x86)	1m 50s
build-and-test / cmake-ios-arm64	2m 28s
build-and-test / cmake-ios-x86_64	3m 16s
build-and-test / cmake-linux-aarch64	1m 8s
build-and-test / cmake-linux-armhf	2m 6s
build-and-test / cmake-linux-riscv64	5m 41s
build-and-test / cmake-linux-riscv64-rvv	2m 11s
build-and-test / cmake-linux-x86_64	1m 54s
build-and-test / cmake-macos-arm64	1m 22s
build-and-test / cmake-windows-arm64	3m 31s
build-and-test / cmake-windows-x86	5m 4s
build-and-test / cmake-windows-x64	4m 35s
build-and-test / bazel-linux-aarch64-gcc13	1m 14s
	44m 53s

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