Support ONNX Flatten and Gemm with transA, transB attributes

[petrohi]

• Support ONNX Flatten [sc-480]
• Support ONNX transA and transB attributes for Gemm [sc-481]
• Enforce ONNX opset version to be in [9, 10] range [sc-482]

[shortcut-integration[bot]]

This pull request has been linked to:

• Shortcut Story #480: Support ONNX Flatten
• Shortcut Story #481: Support ONNX transA and transB attributes for Gemm

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #482: Enforce ONNX opset version.