Update `dynamic` keys and comments for ONNX exports (#3772)

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/engine/exporter.py

@@ -314,10 +314,10 @@ class Exporter:
         if dynamic:
             dynamic = {'images': {0: 'batch', 2: 'height', 3: 'width'}}  # shape(1,3,640,640)
             if isinstance(self.model, SegmentationModel):
-                dynamic['output0'] = {0: 'batch', 1: 'anchors'}  # shape(1,25200,85)
+                dynamic['output0'] = {0: 'batch', 2: 'anchors'}  # shape(1, 116, 8400)
                 dynamic['output1'] = {0: 'batch', 2: 'mask_height', 3: 'mask_width'}  # shape(1,32,160,160)
             elif isinstance(self.model, DetectionModel):
-                dynamic['output0'] = {0: 'batch', 1: 'anchors'}  # shape(1,25200,85)
+                dynamic['output0'] = {0: 'batch', 2: 'anchors'}  # shape(1, 84, 8400)
 
         torch.onnx.export(
             self.model.cpu() if dynamic else self.model,  # --dynamic only compatible with cpu