Validate shapes when updating edges from Python.

Uses MergeInput from shape_inference to check if the new input is compatible with the preexisting shape.  Also this changes the MergeInput method.  Previously, MergeInput would only return true if the shapes differed *and* the merge was successful.  Now, MergeInput returns true only if the merge is successful.

PiperOrigin-RevId: 175576173