fix the temp allocator for backend (#3490)

Summary:
Pull Request resolved: #3490

Seems like forget to pass temp allocator to `BackendExecutionContext`...

Reviewed By: tarun292

Differential Revision: D56893727

fbshipit-source-id: 81bb1208a163220f9b4250493b06069e6329628e

Author: cccclai

runtime/executor/method.cpp

@@ -1063,7 +1063,9 @@ Error Method::execute_instruction() {
           delegate_idx,
           n_delegate_,
           step_state_.instr_idx);
-      BackendExecutionContext backend_execution_context(event_tracer_);
+      BackendExecutionContext backend_execution_context(
+          /*event_tracer*/ event_tracer_,
+          /*temp_allocator*/ memory_manager_->temp_allocator());
       err = delegates_[delegate_idx].Execute(
           backend_execution_context,
           chain.argument_lists_[step_state_.instr_idx].data());