torch.split() 1.7.0 compatibility fix (ultralytics#7102)

* Update loss.py

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* Update loss.py

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

utils/loss.py

@@ -108,13 +108,15 @@ def __init__(self, model, autobalance=False):
         if g > 0:
             BCEcls, BCEobj = FocalLoss(BCEcls, g), FocalLoss(BCEobj, g)
 
-        det = de_parallel(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, 0.02])  # P3-P7
-        self.ssi = list(det.stride).index(16) if autobalance else 0  # stride 16 index
+        m = de_parallel(model).model[-1]  # Detect() module
+        self.balance = {3: [4.0, 1.0, 0.4]}.get(m.nl, [4.0, 1.0, 0.25, 0.06, 0.02])  # P3-P7
+        self.ssi = list(m.stride).index(16) if autobalance else 0  # stride 16 index
         self.BCEcls, self.BCEobj, self.gr, self.hyp, self.autobalance = BCEcls, BCEobj, 1.0, h, autobalance
+        self.na = m.na  # number of anchors
+        self.nc = m.nc  # number of classes
+        self.nl = m.nl  # number of layers
+        self.anchors = m.anchors
         self.device = device
-        for k in 'na', 'nc', 'nl', 'anchors':
-            setattr(self, k, getattr(det, k))
 
     def __call__(self, p, targets):  # predictions, targets
         lcls = torch.zeros(1, device=self.device)  # class loss
@@ -129,7 +131,8 @@ def __call__(self, p, targets):  # predictions, targets
 
             n = b.shape[0]  # number of targets
             if n:
-                pxy, pwh, _, pcls = pi[b, a, gj, gi].tensor_split((2, 4, 5), dim=1)  # target-subset of predictions
+                # pxy, pwh, _, pcls = pi[b, a, gj, gi].tensor_split((2, 4, 5), dim=1)  # faster, requires torch 1.8.0
+                pxy, pwh, _, pcls = pi[b, a, gj, gi].split((2, 2, 1, self.nc), 1)  # target-subset of predictions
 
                 # Regression
                 pxy = pxy.sigmoid() * 2 - 0.5