Consider eliminating explicit `.load_graphdef()` call?

[ludwigschubert]

Hey @colah @znah ;
should we consider automatically calling load_graphdef when instantiating a modelzoo class?
To me this boils down to:

What can you currently do with an instantiated modelzoo.Model, that you couldn't do with just the class?

It feels like this could simplify the current API, but it may also hide the fact that a graph definition may need to be downloaded.
Looking forward to your opinions!

[znah]

Original motivation for having a separate load_graphdef() function was to support some customization, like optional stripping of some layers, or having a few graph versions (say CPU/TPU) for some models. I don't think we really need it now. I support removing it and doing all customization in constructor.

[ludwigschubert]

Thanks for letting me know! Will do a PR soon.