feat(candle-nn): Add RwLock-based VarMap for concurrent inference

[artem1984A]

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[artem1984A]

Applied all suggestions:
Simplified test to use hardcoded expected values (7e3adae)

• Removed comparison with original VarMap implementation
• Tests now directly verify against known expected values
• Makes tests clearer and more maintainable

Used std::env::temp_dir() for cross-platform compatibility (4bc438e)

• Replaced hardcoded /tmp/ paths with OS-agnostic temporary directory
• Now should works correctly on Windows, macOS, Linux, and other platforms

Removed manual memory leak in batch operations (5930869)

• Changed from Box::leak() to Vec<String> + .as_str() pattern