trunk/1c3d446cda0cb4231a810427809d2b3382395e0b: [CUDA] Explain custom MemPool allocator failures in OOM errors (#199363)

What happened

Human note: make a better error message when custom mempool fails allocation Custom allocators signal allocation failure by returning nullptr. The caching allocator maps this to an out-of-memory error, so failures such as invalid allocator configuration can appear to be device-wide memory exhaustion even when the custom allocator logged the underlying cause. Track whether the custom allocator returned nullptr for th…

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