Add custom allocator support to SimpleBlockedGraph (resolves #81)

[devin-ai-integration]

Add custom allocator support to SimpleBlockedGraph (resolves intel#81)

Summary

This PR resolves issue #81 by adding custom allocator support to SimpleBlockedGraph for dynamic vamana index graph construction. Previously, SimpleBlockedGraph was hardcoded to use HugepageAllocator, preventing users from providing their own allocators.

Changes:

• Added allocator template parameter to SimpleBlockedGraph with default HugepageAllocator<Idx> (following the pattern already established in SimpleGraph)
• Added constructor accepting custom allocator: SimpleBlockedGraph(size_t num_nodes, size_t max_degree, const Alloc& allocator)
• Updated both load() methods to accept optional allocator parameter with default
• Added operator== for comparing graphs with different allocators
• Updated three deduction guides in multi.h to explicitly specify HugepageAllocator<uint32_t>

Backward Compatibility:
All existing code continues to work without modification because the allocator template parameter defaults to HugepageAllocator<Idx>. Code using SimpleBlockedGraph<uint32_t> will automatically resolve to SimpleBlockedGraph<uint32_t, HugepageAllocator<uint32_t>>.

Review & Testing Checklist for Human

This PR has moderate risk due to lack of local compilation and runtime testing. Please verify:

• CI compilation succeeds - Verify all tests build without template instantiation errors or type mismatches
• Backward compatibility works - Existing code using SimpleBlockedGraph<uint32_t> should compile and work identically to before
• Deduction guide changes are correct - The three updated deduction guides in multi.h now explicitly specify HugepageAllocator<uint32_t>. Review these carefully to ensure they don't break template argument deduction
• Optional: Test custom allocator usage - Consider manually testing that users can now instantiate SimpleBlockedGraph<uint32_t, CustomAllocator<uint32_t>> and use it with MutableVamanaIndex
• Optional: Add test coverage - Consider adding a test case that explicitly uses a custom allocator with SimpleBlockedGraph to verify the feature works end-to-end

Test Plan Recommendation

1. Run existing test suite to verify backward compatibility
2. Manually instantiate SimpleBlockedGraph with a custom allocator (e.g., std::allocator<uint32_t>) in a test file
3. Verify it can be used with MutableVamanaIndex for graph construction

Notes

• This implementation follows the exact same pattern as SimpleGraph (lines 337-377 in graph.h), which already has custom allocator support
• The change assumes that data::BlockedData properly supports custom allocators (verified from Python bindings usage, but not verified in the actual implementation)
• No new tests were added because existing tests will verify backward compatibility, and the pattern is already proven in SimpleGraph

Devin Session: https://app.devin.ai/sessions/d6b3fe1407f141f3b43e774b3e91aa46
Requested by: @milind-cognition

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