Update usb_semisup_learn.py

Author: Hhhhhhao

advanced_source/usb_semisup_learn.py

@@ -217,7 +217,7 @@
 
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 # References
-# [1] USB
-# [2] FixMatch
-# [3] FreeMatch
-# [4] SoftMatch
+# [1] USB: https://github.com/microsoft/Semi-supervised-learning
+# [2] Kihyuk Sohn et al. FixMatch: Simplifying Semi-Supervised Learning with Consistency and Confidence
+# [3] Yidong Wang et al. FreeMatch: Self-adaptive Thresholding for Semi-supervised Learning
+# [4] Hao Chen et al. SoftMatch: Addressing the Quantity-Quality Trade-off in Semi-supervised Learning