add eigen3 dependency for phi_backends (#44479)

paddle/phi/backends/CMakeLists.txt

 add_subdirectory(dynload)
 
 set(BACKENDS_SRCS all_context.cc cpu/cpu_context.cc)
-set(BACKENDS_DEPS enforce place flags)
+set(BACKENDS_DEPS enforce place flags eigen3)
 
 if(WITH_GPU OR WITH_ROCM)
   list(APPEND BACKENDS_SRCS gpu/gpu_context.cc gpu/gpu_info.cc
@@ -40,6 +40,7 @@ endif()
 
 add_library(phi_backends "${BACKENDS_SRCS}")
 target_link_libraries(phi_backends ${BACKENDS_DEPS})
+add_dependencies(phi_backends eigen3)
 
 # for inference library
 get_property(phi_modules GLOBAL PROPERTY PHI_MODULES)

paddle/phi/backends/custom/CMakeLists.txt

-if(WITH_CUSTOM_DEVICE)
-  cc_library(
-    custom_context
-    SRCS custom_context.cc
-    DEPS phi_device_context device_manager)
-  cc_library(
-    custom_device
-    SRCS custom_device.cc
-    DEPS device_base device_context)
-  cc_test(
-    custom_device_test
-    SRCS custom_device_test.cc
-    DEPS device_manager device_context)
-  cc_test(
-    capi_test
-    SRCS capi_test.cc
-    DEPS phi_capi)
-endif()