Fix compile error on MSVC

See also: https://stackoverflow.com/questions/55136414/constexpr-variable-captured-inside-lambda-loses-its-constexpr-ness

csrc/flash_attn/src/static_switch.h

-// Inspired by https://github.com/NVIDIA/DALI/blob/main/include/dali/core/static_switch.h
+// Inspired by
+// https://github.com/NVIDIA/DALI/blob/main/include/dali/core/static_switch.h
 // and https://github.com/pytorch/pytorch/blob/master/aten/src/ATen/Dispatch.h
 
 #pragma once
@@ -16,10 +17,10 @@
 #define BOOL_SWITCH(COND, CONST_NAME, ...)      \
   [&] {                                         \
     if (COND) {                                 \
-            constexpr bool CONST_NAME = true;                                        \
+      constexpr static bool CONST_NAME = true;  \
       return __VA_ARGS__();                     \
     } else {                                    \
-            constexpr bool CONST_NAME = false;                                       \
+      constexpr static bool CONST_NAME = false; \
       return __VA_ARGS__();                     \
     }                                           \
   }()
@@ -38,28 +39,28 @@
 #define FWD_HEADDIM_SWITCH(HEADDIM, ...)   \
   [&] {                                    \
     if (HEADDIM <= 32) {                   \
-            constexpr int kHeadDim = 32;  \
+      constexpr static int kHeadDim = 32;  \
       return __VA_ARGS__();                \
     } else if (HEADDIM <= 64) {            \
-            constexpr int kHeadDim = 64;  \
+      constexpr static int kHeadDim = 64;  \
       return __VA_ARGS__();                \
     } else if (HEADDIM <= 96) {            \
-            constexpr int kHeadDim = 96;  \
+      constexpr static int kHeadDim = 96;  \
       return __VA_ARGS__();                \
     } else if (HEADDIM <= 128) {           \
-            constexpr int kHeadDim = 128; \
+      constexpr static int kHeadDim = 128; \
       return __VA_ARGS__();                \
     } else if (HEADDIM <= 160) {           \
-            constexpr int kHeadDim = 160; \
+      constexpr static int kHeadDim = 160; \
       return __VA_ARGS__();                \
     } else if (HEADDIM <= 192) {           \
-            constexpr int kHeadDim = 192; \
+      constexpr static int kHeadDim = 192; \
       return __VA_ARGS__();                \
     } else if (HEADDIM <= 224) {           \
-            constexpr int kHeadDim = 224; \
+      constexpr static int kHeadDim = 224; \
       return __VA_ARGS__();                \
     } else if (HEADDIM <= 256) {           \
-            constexpr int kHeadDim = 256; \
+      constexpr static int kHeadDim = 256; \
       return __VA_ARGS__();                \
     }                                      \
   }()