Only look for Caffe2 package when shared (pytorch#18421)

Summary:
Previously it would look for the Config even if it was not written.

Fixed pytorch#18419
Pull Request resolved: pytorch#18421

Differential Revision: D14597139

Pulled By: ezyang

fbshipit-source-id: c212cbf5dc91564c12d9d07e507c8285e11c6bdf

Author: xsacha

cmake/TorchConfig.cmake.in

@@ -36,7 +36,9 @@ else()
 endif()
 
 # Library dependencies.
-find_package(Caffe2 REQUIRED PATHS ${CMAKE_CURRENT_LIST_DIR}/../Caffe2)
+if (@BUILD_SHARED_LIBS@)
+  find_package(Caffe2 REQUIRED PATHS ${CMAKE_CURRENT_LIST_DIR}/../Caffe2)
+endif()
 
 find_library(TORCH_LIBRARY torch PATHS "${TORCH_INSTALL_PREFIX}/lib")
 add_library(torch UNKNOWN IMPORTED)