add examples/siamese_network with triplet loss example

[pax7]

Is your feature request related to a problem? Please describe.

Can you please provide an example of Siamese network training / testing with triplet loss such that it can be used with more complex image datasets?

Describe the solution

Either add an args flag to set triplet loss as the method in the existing example, or provide a separate example for triplet loss.

Describe alternatives solution

I tried to do this on my own.