Add event_tracer to MIL for Backend Init Context (pytorch#11105)

Summary:

Allow for intended usage of event tracer in BackendInitContext.

Reviewed By: cccclai

Differential Revision: D75308778

Author: cmt0

runtime/backend/backend_init_context.h

@@ -25,6 +25,11 @@ class BackendInitContext final {
       const char* method_name = nullptr,
       const NamedDataMap* named_data_map = nullptr)
       : runtime_allocator_(runtime_allocator),
+#ifdef ET_EVENT_TRACER_ENABLED
+      event_tracer_(event_tracer),
+#else
+      event_tracer_(nullptr),
+#endif
         method_name_(method_name),
         named_data_map_(named_data_map) {}