Observer triggers (prologue, epilogue) for module tasks

[nolankramer]

Please describe your feature request.

It's sometimes useful and convenient to capture module performance characteristics (such as scheduling overhead), or trigger callbacks when a module is invoked or finished.

It would be nice if observers also triggered for module tasks. This would allow API callers to do some useful work when aggregate tasks are preparing to run, finishing.

[nolankramer]

As a possible implementation:

inline bool Executor::_invoke_module_task_impl(Worker& w, Node* node, Graph& graph) {
  if(graph.empty()) {
    return false;
  }

  if((node->_nstate & NSTATE::PREEMPTED) == 0) {
    // ADD THIS: Observer sees module start
    _observer_prologue(w, node);
    
    node->_nstate |= NSTATE::PREEMPTED;
    _schedule_graph_with_parent(w, graph.begin(), graph.end(), node->_topology, node);
    return true;
  }

  // ADD THIS: Observer sees module end
  _observer_epilogue(w, node);
  
  node->_nstate &= ~NSTATE::PREEMPTED;
  return false;
}

[tsung-wei-huang]

@nolankramer this is a good suggestion. Would you mind creating a pull request for this? Thanks!