Merge pull request #25810 from fengyuentau:python/fix_parsing_3d_mat_in_dnn

python: attempts to fix 3d mat parsing problem for dnn #25810

Fixes https://github.com/opencv/opencv/issues/25762 https://github.com/opencv/opencv/issues/23242
Relates https://github.com/opencv/opencv/issues/25763 https://github.com/opencv/opencv/issues/19091

Although `cv.Mat` has already been introduced to workaround this problem, people do not know it and it kind of leads to confusion with `numpy.array`. This patch adds a "switch" to turn off the auto multichannel feature when the API is from cv::dnn::Net (more specifically, `setInput`) and the parameter is of type `Mat`. This patch only leads to changes of three places in `pyopencv_generated_types_content.h`:

```.diff
static PyObject* pyopencv_cv_dnn_dnn_Net_setInput(PyObject* self, PyObject* py_args, PyObject* kw)
{
...
- pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 0)) &&
+ pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 8)) &&
...
}

// I guess we also need to change this as one-channel blob is expected for param
static PyObject* pyopencv_cv_dnn_dnn_Net_setParam(PyObject* self, PyObject* py_args, PyObject* kw)
{
...
- pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 0)) )
+ pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 8)) )
...
- pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 0)) )
+ pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 8)) )
...
}
```

Others are unchanged, e.g. `dnn_SegmentationModel` and stuff like that.

### 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
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/python/src2/cv2_convert.hpp

@@ -340,22 +340,38 @@ static bool pyopencv_to_generic_vec(PyObject* obj, std::vector<Tp>& value, const
     {
         return true;
     }
-    if (!PySequence_Check(obj))
+    if (info.nd_mat && PyArray_Check(obj))
     {
-        failmsg("Can't parse '%s'. Input argument doesn't provide sequence protocol", info.name);
-        return false;
-    }
-    const size_t n = static_cast<size_t>(PySequence_Size(obj));
-    value.resize(n);
-    for (size_t i = 0; i < n; i++)
-    {
-        SafeSeqItem item_wrap(obj, i);
-        if (!pyopencv_to(item_wrap.item, value[i], info))
+        /*
+            If obj is marked as nd mat and of array type, it is parsed to a single
+            mat in the target vector to avoid being split into multiple mats
+        */
+        value.resize(1);
+        if (!pyopencv_to(obj, value.front(), info))
         {
-            failmsg("Can't parse '%s'. Sequence item with index %lu has a wrong type", info.name, i);
+            failmsg("Can't parse '%s'. Array item has a wrong type", info.name);
             return false;
         }
     }
+    else // parse as sequence
+    {
+        if (!PySequence_Check(obj))
+        {
+            failmsg("Can't parse '%s'. Input argument doesn't provide sequence protocol", info.name);
+            return false;
+        }
+        const size_t n = static_cast<size_t>(PySequence_Size(obj));
+        value.resize(n);
+        for (size_t i = 0; i < n; i++)
+        {
+            SafeSeqItem item_wrap(obj, i);
+            if (!pyopencv_to(item_wrap.item, value[i], info))
+            {
+                failmsg("Can't parse '%s'. Sequence item with index %lu has a wrong type", info.name, i);
+                return false;
+            }
+        }
+    }
     return true;
 }