Update train.py (#4136)

* Refactor train.py

* Update imports

* Update imports

* Update optimizer

* cleanup

utils/loss.py

@@ -108,7 +108,7 @@ class ComputeLoss:
         det = model.module.model[-1] if is_parallel(model) else model.model[-1]  # Detect() module
         self.balance = {3: [4.0, 1.0, 0.4]}.get(det.nl, [4.0, 1.0, 0.25, 0.06, .02])  # P3-P7
         self.ssi = list(det.stride).index(16) if autobalance else 0  # stride 16 index
-        self.BCEcls, self.BCEobj, self.gr, self.hyp, self.autobalance = BCEcls, BCEobj, model.gr, h, autobalance
+        self.BCEcls, self.BCEobj, self.gr, self.hyp, self.autobalance = BCEcls, BCEobj, 1.0, h, autobalance
         for k in 'na', 'nc', 'nl', 'anchors':
             setattr(self, k, getattr(det, k))