Merge pull request #26875 from asmorkalov:as/in_memory_models

Added trackers factory with pre-loaded dnn models #26875

Replaces https://github.com/opencv/opencv/pull/26295

Allows to substitute custom models or initialize tracker from in-memory model.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [ ] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/video/misc/python/test/test_tracking.py

@@ -7,12 +7,30 @@ from tests_common import NewOpenCVTests, unittest
 
 class tracking_test(NewOpenCVTests):
 
-    def test_createTracker(self):
-        t = cv.TrackerMIL_create()
-        try:
-            t = cv.TrackerGOTURN_create()
-        except cv.error as e:
-            pass  # may fail due to missing DL model files
+    def test_createMILTracker(self):
+        t = cv.TrackerMIL.create()
+        self.assertTrue(t is not None)
+
+    def test_createGoturnTracker(self):
+        proto = self.find_file("dnn/gsoc2016-goturn/goturn.prototxt", required=False);
+        weights = self.find_file("dnn/gsoc2016-goturn/goturn.caffemodel", required=False);
+        net = cv.dnn.readNet(proto, weights)
+        t = cv.TrackerGOTURN.create(net)
+        self.assertTrue(t is not None)
+
+    def test_createNanoTracker(self):
+        backbone_path = self.find_file("dnn/onnx/models/nanotrack_backbone_sim_v2.onnx", required=False);
+        neckhead_path = self.find_file("dnn/onnx/models/nanotrack_head_sim_v2.onnx", required=False);
+        backbone = cv.dnn.readNet(backbone_path)
+        neckhead = cv.dnn.readNet(neckhead_path)
+        t = cv.TrackerNano.create(backbone, neckhead)
+        self.assertTrue(t is not None)
+
+    def test_createVitTracker(self):
+        model_path = self.find_file("dnn/onnx/models/vitTracker.onnx", required=False);
+        model = cv.dnn.readNet(model_path)
+        t = cv.TrackerVit.create(model)
+        self.assertTrue(t is not None)
 
 
 if __name__ == '__main__':