Disable float16 packet casting if native AVX512 f16 is available.

Eigen/src/Core/arch/AVX512/TypeCasting.h

diff --git a/Eigen/src/Core/arch/AVX512/TypeCasting.h b/Eigen/src/Core/arch/AVX512/TypeCasting.h
index 56a94f4..ccdb563 100644
--- a/Eigen/src/Core/arch/AVX512/TypeCasting.h
+++ b/Eigen/src/Core/arch/AVX512/TypeCasting.h
@@ -37,10 +37,12 @@
 template <>
 struct type_casting_traits<int, double> : vectorized_type_casting_traits<int, double> {};
 
+#ifndef EIGEN_VECTORIZE_AVX512FP16
 template <>
 struct type_casting_traits<half, float> : vectorized_type_casting_traits<half, float> {};
 template <>
 struct type_casting_traits<float, half> : vectorized_type_casting_traits<float, half> {};
+#endif
 
 template <>
 struct type_casting_traits<bfloat16, float> : vectorized_type_casting_traits<bfloat16, float> {};