Update train.py (#4136)

* Refactor train.py

* Update imports

* Update imports

* Update optimizer

* cleanup

utils/general.py

@@ -301,7 +301,7 @@ def clean_str(s):
 
 
 def one_cycle(y1=0.0, y2=1.0, steps=100):
-    # lambda function for sinusoidal ramp from y1 to y2
+    # lambda function for sinusoidal ramp from y1 to y2 https://arxiv.org/pdf/1812.01187.pdf
     return lambda x: ((1 - math.cos(x * math.pi / steps)) / 2) * (y2 - y1) + y1