UPSTREAM PR #18357: feat: Add memory factory hook for custom KV cache implementations

[loci-dev]

Mirrored from ggml-org/llama.cpp#18357

Summary

Add a minimal extension point for custom memory (KV cache) implementations.

Motivation

• KV cache optimization is an active research area (compression, semantic caching, etc.)
• Currently requires forking llama.cpp to experiment with custom implementations
• GGML backends already use similar factory patterns

Changes

• Add llama_memory_factory_fn typedef to llama.h
• Add llama_set_memory_factory() to set custom factory
• Check factory before default memory creation in llama_context constructor

Usage

1. Implement factory function returning llama_memory_t
2. Call llama_set_memory_factory() before llama_init_from_model()
3. Factory can return nullptr to use default implementation

Example

static llama_memory_t my_cache_factory(
    const struct llama_model * model,
    const struct llama_context_params * params,
    void * user_data
) {
    if (should_use_custom_cache(params)) {
        return create_my_custom_cache(model, params);
    }
    return nullptr;  // Fall back to default
}

// Register before context creation
llama_set_memory_factory(my_cache_factory, nullptr);

Impact

• Zero overhead when not used (single null pointer check)
• No breaking changes to existing API
• 35 lines total across 2 files