Implement triplet_semihard_loss

[facaiy]

Related with: #26

[seanpmorgan]

Looks great, thank you again. Triplet loss is a great thing to have included in our first package. I like the registering decorator as well... I'm fine with it living in //tensorflow_addons/util.py Also, I think we can decorate all the methods with @tf.function without issue.

When time allows could you decorate the methods as a tf.function, and move the custom object register. Then should be ready to merge.

[facaiy]

I'd like to create a new PR later to move the custom object register, what do you think?

And I agree that we can decorate all the methods with @tf.function. I'm not sure whether it's right time to do it, however, because keras.losses hasn't decorated its functions. By the way, if the base class Loss uses tf.function for its __call__ method in the future, it seems not necessary?

[seanpmorgan]

No issue with a separate PR for moving the register.. it'll need to be applied to all our keras objects anyways.

For the function decorator, I don't see a reason we should hold back just because it's not in tf.keras yet. We're building against tf2-nightly and adapting to TF2 best practices so I think we should include it. The base class implementing @tf.function on __call__ is a good point... but then are all of these other methods such as pairwise_distance never meant to be called outside of Triplet Loss? If so, I would prefer them to have an underscore prefix (But I can imagine it could be used outside of this loss function)

[facaiy]

No issue with a separate PR for moving the register.. it'll need to be applied to all our keras objects anyways.

+1

We're building against tf2-nightly and adapting to TF2 best practices so I think we should include it.

Make sense, I agree with you. And I'll decorate the methods with tf.function in a few days (the day after tomorrow), as I need to attend a wedding tomorrow :-)

[seanpmorgan]

Getting a failing test TripletSemiHardLossTest.test_unweighted due to this commit tensorflow/tensorflow@4604d1e

After that good to merge.

[facaiy]

Fixed. Thank you :-)