Support input shape and ONNX frontend improvements

[petrohi]

• Support ONNX Add with consts
• Support auto_pad in ONNX Conv
• Support freezing input shapes with compiler command-line argument. For example for a model with two input images both having [?, ?, ?, 3] unfrozen shape -i "input_1 [1, 224, 224, 3], input_2 [1, 112, 112, 3]" will freeze to batch size of 1 and 224 by 224 and 112 by 112 image resolution correspondingly. The default for the argument is [1], which will freeze batch size to 1.

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #446: Support input shape and ONNX frontend improvements.