Rnn make stepnet member

[Superjomn]

fix: #3457

[Superjomn]

fix: #3457

[wangkuiyi]

Why would we need to define stepnet_ as a std::shared_ptr? Can we define

class RecurrentOp ... {
 private:
  NetOp step_;

[Superjomn]

In pybind.cc, a std::shared_ptr is passed to RecurrentOp as an argument, so here is a shared pointer too.

[wangkuiyi]

Why do we need shared_ptr in our Python binding?

[Superjomn]

just like NetOp, RNNOp has its own python binding to expose some methods, such as set_stepnet.

In pybind.cc, all the operators created by user are handled by shared_ptr, maybe easier to be referenced across different levels, when user create a Operator, a shared_ptr is returned, it is free to use the pointer, or pass the pointer into multiple NetOp (add an op into a net).

Operators will live across the whole execution period, so a shared_ptr seems reasonable.

[wangkuiyi]

Do you mean something like here:

py::class_<operators::NetOp, std::shared_ptr<operators::NetOp>> net(m, "Net");

It seem that calling Python new Net() returns a shared_ptr<operator::NetOp> -- however, when will the reference count become 0 and the C++ instance get freed?

[wangkuiyi]

Why unique_ptr here? Could we just do

class RecurrentOp ... {
 private:
  rnn::Argument arg_;

[Superjomn]

the rnn::Argument is a argument that should be passed to RecurrentAlgorithm, a unique pointer may be lighter to pass, and std::move is used here, finally only RecurrentAlgorithm owns this memory.

[wangkuiyi]

I don't get it. By defining arg_ a data member of RecurrentOp, we can of course call RecurrentAlgorithm(&arg_);, could we?

[Superjomn]

get it.