Fix codegen writing config.h into source tree

[alexioslyrakis-amd]

Summary

codegen/CMakeLists.txt uses configure_file() to generate config.h directly into the source tree (include/ck/config.h) instead of the build directory. This causes -Wmacro-redefined warnings because:

1. The generated source-tree config.h unconditionally #defines macros like CK_ENABLE_INT8, CK_ENABLE_FP8, etc.
2. The same macros are also passed via -D flags on the command line by CMake
3. The source-tree include/ is searched before the build-dir include/, so this guardless file wins over the properly generated one

The main CMakeLists.txt (line 530) already generates config.h correctly into ${CMAKE_CURRENT_BINARY_DIR}. This PR fixes codegen/CMakeLists.txt to do the same.

Example warnings

In file included from .../ck/include/ck/ck.hpp:6:
.../ck/include/ck/config.h:5:9: warning: 'CK_ENABLE_INT8' macro redefined [-Wmacro-redefined]
    5 | #define CK_ENABLE_INT8 "ON"
      |         ^
<command line>:8:9: note: previous definition is here
    8 | #define CK_ENABLE_INT8 1
      |         ^

.../ck/include/ck/config.h:6:9: warning: 'CK_ENABLE_FP8' macro redefined [-Wmacro-redefined]
    6 | #define CK_ENABLE_FP8 "ON"
      |         ^
<command line>:7:9: note: previous definition is here
    7 | #define CK_ENABLE_FP8 1
      |         ^

.../ck/include/ck/config.h:8:9: warning: 'CK_ENABLE_FP16' macro redefined [-Wmacro-redefined]
    8 | #define CK_ENABLE_FP16 "ON"
      |         ^
<command line>:4:9: note: previous definition is here
    4 | #define CK_ENABLE_FP16 1
      |         ^

These warnings repeat for every translation unit that includes ck.hpp.

Fix

Change codegen/CMakeLists.txt to write the generated config.h to ${CMAKE_CURRENT_BINARY_DIR} instead of the source tree, matching the main CMakeLists.txt.