Allow proceeding without --input_shape flags when the model already has that...

Allow proceeding without --input_shape flags when the model already has that information (e.g. when the inputs are Placeholders with shape attribs).
This required moving the call to ResolveModelFlags back until after operators are imported (where it was until it was moved recently for drop_control_dependency handling).

PiperOrigin-RevId: 176699182