Fix op_add with constant

[xsacha]

Fixes #26

[hakanardo]

Please consider adding a test aswell.

[hakanardo]

This could break things as constants might appear in other ops than add. How about keeping the old solution but add an inefficiency warning to it and then we can fix the ops one by one as needed?

[xsacha]

Sorry, I'm unsure about what you mean there. Without this change, the model won't load at all.
For the OnnxConstant, is there an example that fails when b has no size? I can't think of an example for that test.

[xsacha]

Any update on this?
Is there a corner case I need to test? Is there an alternative fix for #26?

Thanks