ONNX export in .train() mode fix (#3362)

models/export.py

@@ -97,6 +97,8 @@ if __name__ == '__main__':
             print(f'{prefix} starting export with onnx {onnx.__version__}...')
             f = opt.weights.replace('.pt', '.onnx')  # filename
             torch.onnx.export(model, img, f, verbose=False, opset_version=opt.opset_version, input_names=['images'],
+                              training=torch.onnx.TrainingMode.TRAINING if opt.train else torch.onnx.TrainingMode.EVAL,
+                              do_constant_folding=not opt.train,
                               dynamic_axes={'images': {0: 'batch', 2: 'height', 3: 'width'},  # size(1,3,640,640)
                                             'output': {0: 'batch', 2: 'y', 3: 'x'}} if opt.dynamic else None)