`remove_unused_nodes`: removing inputs

[leshabirukov]

Hello again!
While processing some net, I ended up with strange picture with hanging inputs like this:

It appears, where was initializers dubbed as inputs in the "some net". After rewriting, those become unused, and call to remove_unused_nodes removed initializers, but keeped inputs.
I don't fully understand what 'initializers as inputs' mean, but it looks rather legal, and Netron and onnxruntime.checker accepts them. So, it is probably make sense remove unused inputs also.

[leshabirukov]

In the same time, inputs and outputs can be seen as part of net's interface and agreement between net and outer world, so it should remain. The question is, does it extends to ininitalizer inputs?