feat: add automatic direct convolution selection

[leejet]

Summary

• Add --conv-direct on|off|auto and conv_direct_mode with per-module overrides for diffusion, VAE, and ControlNet.
• Automatically enable supported implicit GEMM paths using capability probes without benchmarking. Cache 2D/3D decisions per backend and log selection reasons.
• Deprecate the legacy convolution flags while preserving compatibility, and document the new behavior.

Related Issue / Discussion

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Additional Information

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Checklist

• I have read and confirmed this PR follows the contribution guidelines.

Summary by CodeRabbit

• New Features
  • Added a --conv-direct on|off|auto setting, with optional per-module overrides for diffusion, VAE, and ControlNet.
  • In auto mode, direct convolution is enabled only when the runtime backend supports it. Mixed backend support is reported, and legacy convolution flags are honored when the new setting is unset.
• Documentation
  • Updated backend and CLI guides with the new setting, defaults, supported modules, and backend behavior.
• Compatibility
  • Legacy convolution flags remain available but now produce deprecation warnings.

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📥 Commits

Reviewing files that changed from the base of the PR and between d763dae and 1b4c27f.

📒 Files selected for processing (15)

• docs/backend.md
• docs/backend_probe.md
• examples/cli/README.md
• examples/common/common.cpp
• examples/common/common.h
• include/stable-diffusion.h
• src/core/ggml_extend_backend.cpp
• src/core/ggml_extend_backend.h
• src/core/probe/conv_direct.cpp
• src/core/probe/conv_direct.h
• src/model_manager.cpp
• src/pipeline/diffusion_engine.cpp
• src/pipeline/diffusion_engine.h
• src/pipeline/model_builders.cpp
• src/stable-diffusion.cpp

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