add ref for emlloss

ppcls/loss/emlloss.py

@@ -23,6 +23,11 @@ from .comfunc import rerange_index
 
 
 class EmlLoss(paddle.nn.Layer):
+    """Ensemble Metric Learning Loss
+    paper: [Large Scale Strongly Supervised Ensemble Metric Learning, with Applications to Face Verification and Retrieval](https://arxiv.org/pdf/1212.6094.pdf)
+    code reference: https://github.com/PaddlePaddle/models/blob/develop/PaddleCV/metric_learning/losses/emlloss.py
+    """
+
     def __init__(self, batch_size=40, samples_each_class=2):
         super(EmlLoss, self).__init__()
         assert (batch_size % samples_each_class == 0)