Bump onnx from 1.18.0 to 1.21.0rc1 in /onnxruntime/python/tools/transformers/models/stable_diffusion/requirements#1009

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dependabot/pip/onnxruntime/python/tools/transformers/models/stable_diffusion/requirements/onnx-1.21.0rc1intel/onnxruntime:dependabot/pip/onnxruntime/python/tools/transformers/models/stable_diffusion/requirements/onnx-1.21.0rc1Copy head branch name to clipboard