Add model and space link for melgan_spectrogram_inversion, mpnn-molecular-graphs, wgan-graphs

[vumichien]

As per the ongoing hugs keras sprint, have updated the following examples:

Audio Data:

• melgan_spectrogram_inversion

Graph Data:

• mpnn-molecular-graphs

Generative Deep Learning

• wgan-graphs

Will be glad to address any issues/changes.

[google-cla]

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[fchollet]

Thanks for the PR!

Generally speaking, prefer doing one PR per code example. It's easier to review.