v1.21.3: Patch release

• Deprecate ORTModel class by @IlyasMoutawwakil in #1939
• Remove warning by @echarlaix in #1945
• Fix optimizer's command line reading by @idruker-cerence in #1961
• Fix unmask_unattended_patched signature by @fxmarty in #1963
• Fix gpt bigcode ONNX export for transformers<4.39.0 by @echarlaix in #1973
• Support transformers 4.43 by @IlyasMoutawwakil in #1971

Full Changelog: v1.21.2...v1.21.3