Multi array label Softmax Cross Entropy loss function

[deepjavalibrary]

Multi array label Softmax Cross Entropy loss function

[zachgk]

I created an example showing how to implement a multi array label softmax cross entropy loss function at https://github.com/awslabs/djl/blob/master/examples/src/main/java/ai/djl/examples/training/TrainCaptcha.java. Please take a look and let me know if there are any further problems or suggestions.

[frankfliu]

@jSaso
Thanks very much for reaching out to us.

Our currently API already support multiple input/multiple label/multiple output use case.
I agree it's not obvious by just looking at the API. And we don't have examples to show case this usage. We will add examples/document to make it clear.

Here is some quick pointer:

1. Loss API uses NDList, NDList is a list of NDArray, not a single NDArray, so it can handle multiple data and labels.
2. Batch API uses NDList type for data and labels, it's not NDArray, NDList can take multiple data and multiple labels.
3. Translator, when pre-processing and post processing data, you can see its also using NDList, which allows user handle multiple input and outputs.

NDList class not only allows users process NDArray by its order, you can actually use named NDArray, so you don't need worry about the order of the inputs. Here is code that handles multiple inputs: https://github.com/awslabs/djl/blob/master/mxnet/mxnet-model-zoo/src/main/java/ai/djl/mxnet/zoo/nlp/qa/BertQATranslator.java#L53-L62

[jSaso]

Thanks @frankfliu
I saw that API uses NDList. Thats why I thoot that multi array labels - multi class are supported. This is not the case in current API.
It would be nice to to have this supported - implement softmax loss function which support not just single label array, but multiple labels array.
Currently I don't have time to open PR about this feature, but it would be nice to have that feature in DJL API.

[zachgk]

@jSaso Could you use the CompositeLoss to combine multiple SoftmaxCrossEntropyLosses together?