Add indirect connection support

[hfhongzy]

This PR introduces a new feature called indirect connection, enabling data transfer from endpoint A to endpoint C via forwarding through endpoint B. It defines two abstractions: buffer resources (send and receive buffers on the forwarding endpoint) and scheduler (which manages the data movement logic). We provide an implementation based on Vortex (Yichao, Lin, and Nishil. https://arxiv.org/abs/2502.09541), which uses a toggling double buffer and a pipeline style to overlap the two transfer paths efficiently.

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[Binyang2014]

Thanks @hfhongzy, let me go through the paper first. Since we introduced VortexScheduler, thinking if we could have a more general implementation for this kind of requirements

[Binyang2014]

Hi @hfhongzy, I have a question for your PR. From your implementation, seems you can create your own connection by inherits from the Connection class. So, do we really need to change MSCCL++ code? I mean if you can implement similar thing in your code repo, such as:

namespace your_namespace {
class IndirectConnection  : public mscclpp::Connection {
...
};
};

Then create the connection via auto conn = IndirectConnection(ctx, localEndpoint, scheduler_ptr)

I think MSCCL++ should support customized p2p connection, trying to better understand your requirements

[hfhongzy]

Hi @Binyang2014, thanks for your reply! I believe we (University of Michigan) have been discussing with Changho @chhwang about adding this feature in MSCCL++. Apologies for the confusion here.