Bugfix: training header files were not installed with "make install". (#18004)

Fixes bug reported here:
#17979

These files were not installed with "make install":
    executorch/extension/training/module/state_dict_util.h
    executorch/extension/training/module/training_module.h
    executorch/extension/training/optimizer/sgd.h

 This commit fixes this by adding the appropriate install commands.

Adding @JacobSzwejbka to the conversation.

Co-authored-by: Nikhil Viswanath Sivakumar <68182521+nil-is-all@users.noreply.github.com>

Author: berndporr

extension/training/CMakeLists.txt

@@ -94,3 +94,18 @@ install(
   INCLUDES
   DESTINATION ${_common_include_directories}
 )
+
+# Headers
+install(
+  DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/module
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/extension/training
+  FILES_MATCHING
+  PATTERN "*.h"
+)
+
+install(
+  DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/optimizer
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/extension/training
+  FILES_MATCHING
+  PATTERN "*.h"
+)