Install the sentencepiece headers and fix include path

I'm not sure exactly why, but we weren't installing the sentencepiece headers. We also need to set our include path to point at our installed headers.

Author: swolchok

CMakeLists.txt

@@ -82,6 +82,7 @@ target_include_directories(
   tokenizers
   PUBLIC
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/pytorch/tokenizers>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/third-party/sentencepiece>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/third-party/sentencepiece/src>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/third-party/re2>
@@ -190,6 +191,13 @@ install(
   PATTERN "*.h"
 )
 
+install(
+  DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/third-party/sentencepiece/src/
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/pytorch/tokenizers
+  FILES_MATCHING
+  PATTERN "sentencepiece_processor.h"
+  PATTERN "sentencepiece_trainer.h"
+)
 install(
   DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/third-party/json/single_include/
   DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}