[ARM CPU] Fix fp16 const initialization on no-fp16 platform (#23978)

### Description
Fix fp16 const initialization on no-fp16 platform [such as Raspberry
PI](#23957)



### Motivation and Context
Resolve #23957

Author: fajin-corp

onnxruntime/core/mlas/lib/softmax_kernel_neon_fp16.cpp

@@ -20,7 +20,6 @@ Module Name:
 #include "softmax.h"
 #include "softmax_kernel_neon.h"
 
-// TODO(fajin): intra-loop parallelism
 namespace softmax_neon {
 
 template <typename T>
@@ -44,7 +43,7 @@ struct MlasExpConstants {
     T MaximumExponent;
 };
 
-const MlasExpConstants<_mlas_fp16_> ExpConstantsFp16 = {
+constexpr MlasExpConstants<_mlas_fp16_> ExpConstantsFp16 = {
     0xcc55, // -25 * ln2
     0x498c, // 16 * ln2
     0xc95f, // -15.5 * ln2
@@ -64,67 +63,65 @@ const MlasExpConstants<_mlas_fp16_> ExpConstantsFp16 = {
     0x3C00, // 15
 };
 
-const MlasExpConstants<float16x8_t> ExpConstantsFp16x8 = {
-    MlasBroadcastFloat16x8(ExpConstantsFp16.LowerRange),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.UpperRange),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.LowerRangeSumExp),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.UpperRangeSumExp),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.RoundingBias),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.Log2Reciprocal),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.Log2High),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.Log2Mid),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.Log2Low),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.poly_0),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.poly_1),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.poly_2),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.poly_3),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.poly_4),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.poly_56),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.MinimumExponent),
-    MlasBroadcastFloat16x8(ExpConstantsFp16.MaximumExponent),
-};
-
-const MlasExpConstants<float16x4_t> ExpConstantsFp16x4 = {
-    MlasBroadcastFloat16x4(ExpConstantsFp16.LowerRange),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.UpperRange),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.LowerRangeSumExp),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.UpperRangeSumExp),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.RoundingBias),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.Log2Reciprocal),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.Log2High),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.Log2Mid),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.Log2Low),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.poly_0),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.poly_1),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.poly_2),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.poly_3),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.poly_4),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.poly_56),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.MinimumExponent),
-    MlasBroadcastFloat16x4(ExpConstantsFp16.MaximumExponent),
-};
-
 template <typename T>
 MLAS_FORCEINLINE
-MlasExpConstants<T> Get_Exp_Constants();
+const MlasExpConstants<T>& Get_Exp_Constants();
 
 template <>
 MLAS_FORCEINLINE
-MlasExpConstants<float16x8_t> Get_Exp_Constants<float16x8_t>() {
+const MlasExpConstants<float16x8_t>& Get_Exp_Constants<float16x8_t>() {
+    const static MlasExpConstants<float16x8_t> ExpConstantsFp16x8 = {
+        MlasBroadcastFloat16x8(ExpConstantsFp16.LowerRange),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.UpperRange),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.LowerRangeSumExp),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.UpperRangeSumExp),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.RoundingBias),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.Log2Reciprocal),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.Log2High),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.Log2Mid),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.Log2Low),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.poly_0),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.poly_1),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.poly_2),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.poly_3),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.poly_4),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.poly_56),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.MinimumExponent),
+        MlasBroadcastFloat16x8(ExpConstantsFp16.MaximumExponent),
+    };
     return ExpConstantsFp16x8;
 }
 
 template <>
 MLAS_FORCEINLINE
-MlasExpConstants<float16x4_t> Get_Exp_Constants<float16x4_t>() {
+const MlasExpConstants<float16x4_t>& Get_Exp_Constants<float16x4_t>() {
+    const static MlasExpConstants<float16x4_t> ExpConstantsFp16x4 = {
+        MlasBroadcastFloat16x4(ExpConstantsFp16.LowerRange),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.UpperRange),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.LowerRangeSumExp),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.UpperRangeSumExp),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.RoundingBias),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.Log2Reciprocal),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.Log2High),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.Log2Mid),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.Log2Low),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.poly_0),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.poly_1),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.poly_2),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.poly_3),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.poly_4),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.poly_56),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.MinimumExponent),
+        MlasBroadcastFloat16x4(ExpConstantsFp16.MaximumExponent),
+    };
     return ExpConstantsFp16x4;
 }
 
 // Range reduction + polynomial approximation. Refer algorithm details to MlasComputeExpVector.
 template<typename T>
 MLAS_FORCEINLINE
 T Exp_Vector_Fp16(T x) {
-    const auto constants = Get_Exp_Constants<T>();
+    const auto& constants = Get_Exp_Constants<T>();
     auto clamped_x = MlasClampFloat16(x, constants.LowerRange, constants.UpperRange);
 
     // integral
@@ -242,7 +239,7 @@ void Exp_Kernel_Fp16(const MLAS_FP16* Input, MLAS_FP16* Output, size_t N) {
 template<typename T>
 MLAS_FORCEINLINE
 T SumExp_Vector_Fp16(T x, T negative_maximum) {
-    const auto constants = Get_Exp_Constants<T>();
+    const auto& constants = Get_Exp_Constants<T>();
     auto clamped_x = MlasMaximumFloat16(MlasAddFloat16(x, negative_maximum), constants.LowerRangeSumExp);
 
     // integral
@@ -419,7 +416,7 @@ struct MlasTanhConstants {
     T beta_0;
 };
 
-const MlasTanhConstants<_mlas_fp16_> TanhConstantsFp16 = {
+constexpr MlasTanhConstants<_mlas_fp16_> TanhConstantsFp16 = {
     0xc308, // -3.51562
     0x4308, // 3.51562
     0x0001,
@@ -432,53 +429,51 @@ const MlasTanhConstants<_mlas_fp16_> TanhConstantsFp16 = {
     0x1d03,
 };
 
-const MlasTanhConstants<float16x8_t> TanhConstantsFp16x8 = {
-    MlasBroadcastFloat16x8(TanhConstantsFp16.LowerRange),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.UpperRange),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_7),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_5),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_3),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_1),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.beta_6),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.beta_4),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.beta_2),
-    MlasBroadcastFloat16x8(TanhConstantsFp16.beta_0),
-};
-
-const MlasTanhConstants<float16x4_t> TanhConstantsFp16x4 = {
-    MlasBroadcastFloat16x4(TanhConstantsFp16.LowerRange),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.UpperRange),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_7),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_5),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_3),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_1),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.beta_6),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.beta_4),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.beta_2),
-    MlasBroadcastFloat16x4(TanhConstantsFp16.beta_0),
-};
-
 template <typename T>
 MLAS_FORCEINLINE
-MlasTanhConstants<T> Get_Tanh_Constants();
+const MlasTanhConstants<T>& Get_Tanh_Constants();
 
 template <>
 MLAS_FORCEINLINE
-MlasTanhConstants<float16x8_t> Get_Tanh_Constants<float16x8_t>() {
+const MlasTanhConstants<float16x8_t>& Get_Tanh_Constants<float16x8_t>() {
+    const static MlasTanhConstants<float16x8_t> TanhConstantsFp16x8 = {
+        MlasBroadcastFloat16x8(TanhConstantsFp16.LowerRange),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.UpperRange),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_7),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_5),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_3),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.alpha_1),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.beta_6),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.beta_4),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.beta_2),
+        MlasBroadcastFloat16x8(TanhConstantsFp16.beta_0),
+    };
     return TanhConstantsFp16x8;
 }
 
 template <>
 MLAS_FORCEINLINE
-MlasTanhConstants<float16x4_t> Get_Tanh_Constants<float16x4_t>() {
+const MlasTanhConstants<float16x4_t>& Get_Tanh_Constants<float16x4_t>() {
+    const static MlasTanhConstants<float16x4_t> TanhConstantsFp16x4 = {
+        MlasBroadcastFloat16x4(TanhConstantsFp16.LowerRange),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.UpperRange),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_7),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_5),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_3),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.alpha_1),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.beta_6),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.beta_4),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.beta_2),
+        MlasBroadcastFloat16x4(TanhConstantsFp16.beta_0),
+    };
     return TanhConstantsFp16x4;
 }
 
 // TODO(fajin): optimize polynomial coefficients
 template <typename T>
 MLAS_FORCEINLINE
 T Tanh_Vector_Fp16(T x) {
-    const auto constants = Get_Tanh_Constants<T>();
+    const auto& constants = Get_Tanh_Constants<T>();
     x = MlasClampFloat16(x, constants.LowerRange, constants.UpperRange);
 
     T x_2 = MlasMultiplyFloat16(x, x);