add apply softmax option to ClassificationModel

[UnaNancyOwen]

Fix #21689

// model doesn't contain softmax layer
cv::dnn::ClassificationModel classification_model = cv::dnn::ClassificationModel( model, config );

// not apply softmax post process (default)
bool apply_softmax = classification_model.getSoftmaxPostProcessing(); // false

// apply softmax post process
apply_softmax = true;
classification_model.setEnableSoftmaxPostProcessing( apply_softmax );

// if enable softmax post process is true, confidences range is [0.0-1.0]
// if enable softmax post process is false, confidences range is varies by model
auto [classid, confidence] = model.classify( image );

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[alalek]

Thank you for contribution!

[UnaNancyOwen]

dropped idea
@alalek Should define softmax() function outside of the ClassficationModel class?
It is considered to be a general purpose function on the same level as NMSBoxes.
What do you think about this idea?

// Run Softmax defined inside ClassficationModel class
cv::dnn::ClassficationModel::softmax(input_blob, output_blob);

// Run Softmax defined outside ClassficationModel class
cv::dnn::softmax(input_blob, output_blob);

[alalek]

Well done! Thank you for contribution 👍

[vpisarev]

suspicious place