Exposing Channel to be used as a Variable and integrating with Fluid

[kavyasrinet]

Fixes : #8485 and #8484

@abhinavarora and I have been pair programming to expose the Channel class to be used a Variable and integrating it with the rest of Fluid framework.

[wangkuiyi]

It looks to me that it might be possible to remove the current base class Channel from channel.h, and rename ChannelHolder into Channel, if we write here

std::unique_ptr<
    boost::variant<
        BufferedChannel<T>, 
        UnbufferedChannel<T>>> channel_;

I am not an expert of boost::variant, but I see it is used many places in our codebase. For example:

Paddle/paddle/fluid/platform/place.h

Line 53 in 24509f4

typedef boost::variant<CUDAPlace, CPUPlace> Place;

[wangkuiyi]

I verified that we could use the polymorphism provided by boost::variant to call Send:

#include <iostream>
#include <boost/variant.hpp>

class BufferedChannel {
public:
  bool Send() {
    std::cout << "BufferedChannel::Send\n";
    return false;
  }
};

class UnbufferedChannel {
public:
  bool Send() {
    std::cout << "UnbufferedChannel::Send\n";
    return false;
  }
};

class SendVisitor : public boost::static_visitor<bool> {
public:
  template <typename T>
  bool operator()(T& t) const {
    return t.Send();
  }
};

int main() {
  boost::variant<BufferedChannel, UnbufferedChannel> ch;
  BufferedChannel bch;
  ch = bch;
  boost::apply_visitor(SendVisitor(), ch);
  return 0;
}

[abhinavarora]

Thank you @wangkuiyi . This looks interesting. I'll look into this Visitor pattern polymorphism.

[wangkuiyi]

Extend the above example to work with unique_ptr:

#include <iostream>
#include <boost/variant.hpp>

class BufferedChannel {
public:
  bool Send() {
    std::cout << "BufferedChannel::Send\n";
    return false;
  }
};

class UnbufferedChannel {
public:
  bool Send() {
    std::cout << "UnbufferedChannel::Send\n";
    return false;
  }
};

class SendVisitor : public boost::static_visitor<bool> {
public:
  template <typename T>
  bool operator()(T* t) const {
    return t->Send();
  }
};

int main() {
  typedef boost::variant<BufferedChannel*, UnbufferedChannel*> ChannelVariant;
  std::unique_ptr<ChannelVariant> ch;
  ch.reset(new ChannelVariant(new BufferedChannel));
  boost::apply_visitor(SendVisitor(), *ch.get());
  return 0;
}

[wangkuiyi]

I am open to merge this PR and then try to remove a level of abstraction -- remove class Channel or remove class ChannelHolder.

[kavyasrinet]

Thank you so much for your comments @wangkuiyi , we are looking into doing it the boost::variant way as you explained. We can do as you suggest -- merge this PR and create a new one once we unify Channel and ChannelHolder.