[ENH] Improve how we check for MG path at the C++ level

[afender]

In several places we are using comms initialization as a way to specify whether to take the MG path or not:
if (handle.comms_initialized()) {

We need a better way to separate MG distributed from MG batch other than checking the handle communicator. We should also keep consolidation in mind. There are several things we should do :

1. Add a batch parameter at the C++ API level, this is something we could leverage.
2. template parameter to enables compile-time resolution of multi-GPU vs single-GPU.

template <bool multi_gpu = false, ...>
class Graph {
  static constexpr bool is_multi_gpu = multi_gpu;
  ...
};

3. When a distributed graph is constructed we fill some specific structures. We should also set an "is_distributed" status in the graph that analytics can check this.

I think we should add both 1. and 2. first and consider 3. if 2. has complications at the python layer

[BradReesWork]

moving to 0.16 and the big push for better MG support

[seunghwak]

I think this issue is obsolete now.

This is relevant to only 1D BFS & PageRank, but those will retire by the end of 0.16.

Addressing this requires updating the old graph class and can require sizable amount of effort, and I don't think this is really worth the effort.

This is already part of the new graph class and the pattern accelerator API based BFS & PageRank implementations.

[rlratzel]

To summarize the effort so far, I added the bool template param to the graph classes in C++, but passing a non-type template arg in Cython was causing problems. I tried using a workaround described here, which made Cython happy, but the resulting C++ code looked suspicious and didn't compile, so I don't know if I used it correctly (still looking into that). Here's the generated code from the workaround for anyone interested:

/* "cugraph/structure/graph_new.pxd":24
 * from rmm._lib.device_buffer cimport device_buffer
 *
 * ctypedef bool falseParamVal "false"             # <<<<<<<<<<<<<<
 * ctypedef bool trueParamVal "true"
 *
 */
typedef bool false;

@Iroy30 pointed me to a thread with some additional ideas here, which I have yet to try. Unfortunately it appears that formal support for non-type template args in Cython still hasn't been added yet, based on this still-open PR

Another consideration is that since this is an additional template arg to the graph classes, it means that every Cython file will need to be updated, making it a fairly intrusive PR. I think that's normally okay, but as @seunghwak mentioned, it might be outside the intended scope of the issue if we already have it addressed in a new pattern accelerator-based approach to be applied soon.

[BradReesWork]

This issue is really about the old 1D structure and is now OBE. Closing