Simplify SAM 2.1 decoder output and fix ONNX export warnings

[andriiryzhkov]

• Remove mask upscaling from SAM 2.1 decoder ONNX export - output masks at native 256x256 instead of 1024x1024, drop low_res_masks output. Resizing to target dimensions is handled on the C side.
• Fix onnxruntime MergeShapeInfo warnings by adding onnxsim to dependencies and stripping empty shape annotations after graph simplification.
• Update all SAM 2.1 demos to match new decoder output format (2 outputs instead of 3).

[TurboGit]

Thanks!