[cuda.core] Add support for Multicast Objects

Feature Request

Add Python bindings and a cuda.core abstraction for CUDA Multicast Objects
(the cuMulticastCreate / cuMulticastAddDevice / cuMulticastBindMem /
cuMulticastBindAddr / cuMulticastUnbind family of driver APIs).

Motivation

Multicast Objects enable a single virtual address to multicast memory accesses
across multiple devices, which is foundational for efficient multi-GPU
collectives and SHARP-style reductions on NVLink-connected systems (Hopper+
and beyond). They are already exposed via the low-level cuda.bindings driver
API, but there is no high-level cuda.core object wrapping them.

This is one of the last major CUDA driver features without a cuda.core
counterpart. Internal teams building multi-GPU libraries (RAPIDS, Triton,
cuBLASMp, etc.) currently have to drop to raw driver calls.

Proposed Scope

• A MulticastObject (or similarly named) class with:
  • Constructor / factory taking granularity + participating-device list
  • add_device(device) / bind_memory(buffer) / bind_address(...) /
    unbind(...) methods
  • Context-manager lifecycle (auto-release on exit)
  • Integration with existing Device, Buffer, and VirtualMemoryResource
• Query helpers for multicast granularity (cuMulticastGetGranularity)
• Example(s) under cuda_core/examples/ demonstrating a simple 2-GPU
  multicast allreduce-style pattern
• API reference entry in cuda_core/docs/source/api.rst

Related

• Tracking gaps identified in cuda.core feature audit (Nov 2025) — other
  untracked gaps include host_launch and Library (cuLibrary) APIs; those
  can be filed separately.
• Relevant driver APIs: cuMulticastCreate, cuMulticastAddDevice,
  cuMulticastBindMem, cuMulticastBindAddr, cuMulticastUnbind,
  cuMulticastGetGranularity.