reporting name collisions from rewriter

[leshabirukov]

I have an issue with rewriter, while creating an output value like this:

def ons_init(op, np_array, inzer_name ):  
    assert init_name_is_uniq(op, inzer_name), f'ons_init: name "{inzer_name}" is already used!'
    inzer_help = numpy_helper.from_array(np_array)  
    inzer_irir = TensorProtoTensor(inzer_help)  
    inzer = op.initializer(inzer_irir, inzer_name)  
    return inzer  

maker_3.py.txt

this code is used by 'replacement', cause name collisions. assert init_name_is_uniq not helps, because some substitutions work in parallel, (they see same source graph, accept same inzer_name ) then rewriter do substitutions, resulting one of initializers reused, others killed. It would be great, if rewriter at least reports the error.

A suppose it is rather a bug, considering latter:
#1536

[justinchuby]

cc @gramalingam