OnnxAttributes type conversion is error prone

[ezyang]

The OnnxAttributes helper class that I wrote suffers the problem that it uses exactly the type of the attribute value to decide whether or not to write out f or i attribute protos. I don't know if Caffe2 will accept an i anywhere a f is expected, but if it does not, then this leads to the very real danger that someone writes a literal like 1 or 2, expecting it to "just work", but actually the underlying attribute required floats.