Merge pull request #17502 from dmatveev:dm/infer2

* G-API: Introduce a new gapi::infer2 overload + gaze estimation sample

* G-API/infer2: Introduced static type checking for infer2

- Also added extra tests on the type check routine

* G-API/infer2: Addressed self-review comments in the sample app

- Also fix build on Linux;

* G-API/infer2: Remove incorrect SetLayout(HWC) + dead code

- Also fixed comments in the backend

* G-API/infer2: Continue with self-review

- Fix warnings/compile errors in gaze estimation
- Dropped the use of RTTI (VectorRef::holds()) from the giebackend
- Replaced it with a trait-based enums for GArray<T> and std::vector<T>
- The enums and traits are temporary and need to be unified with
  the S11N when it comes

* G-API/infer2: Final self-review items

- Refactored ROIList test to cover 70% for infer<> and infer2<>;
- Fixed the model data discovery routine to be compatible with new
  OpenVINO;
- Hopefully fixed the final issues (warnings) with the sample.

* G-API/infer2: address review problems

- Fixed typo in comments;
- Fixed public (Doxygen) comment on GArray<GMat> input case for infer2;
- Made model lookup more flexible to allow new & old OMZ dir layouts.

* G-API/infer2: Change the model paths again

* G-API/infer2: Change the lookup path for test data

* G-API/infer2: use randu instead of imread. CI war is over

modules/gapi/test/infer/gapi_infer_tests.cpp

@@ -0,0 +1,79 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+//
+// Copyright (C) 2020 Intel Corporation
+
+#include "../test_precomp.hpp"
+
+// These tests verify some parts of cv::gapi::infer<> API
+// regardless of the backend used
+
+namespace opencv_test {
+namespace {
+template<class A, class B> using Check = cv::detail::valid_infer2_types<A, B>;
+
+TEST(Infer, ValidInfer2Types)
+{
+    // Compiled == passed!
+
+    // Argument block 1
+    static_assert(Check< std::tuple<cv::GMat>   // Net
+                       , std::tuple<cv::GMat> > // Call
+                  ::value == true, "Must work");
+
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::GMat, cv::GMat> > // Call
+                  ::value == true, "Must work");
+
+    // Argument block 2
+    static_assert(Check< std::tuple<cv::GMat>             // Net
+                       , std::tuple<cv::Rect> >           // Call
+                  ::value == true, "Must work");
+
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::Rect, cv::Rect> > // Call
+                  ::value == true, "Must work");
+
+    // Argument block 3 (mixed cases)
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::GMat, cv::Rect> > // Call
+                  ::value == true, "Must work");
+
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::Rect, cv::GMat> > // Call
+                  ::value == true, "Must work");
+
+    // Argument block 4 (super-mixed)
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::Rect, cv::GMat, cv::Rect> > // Call
+                  ::value == true, "Must work");
+
+    // Argument block 5 (mainly negative)
+    static_assert(Check< std::tuple<cv::GMat>             // Net
+                       , std::tuple<int> >                // Call
+                  ::value == false, "This type(s) shouldn't pass");
+
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<int, cv::Rect> >      // Call
+                  ::value == false, "This type(s) shouldn't pass");
+
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::Rect, cv::Point> >// Call
+                  ::value == false, "This type(s) shouldn't pass");
+
+    // Argument block 5 (wrong args length)
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::GMat> >           // Call
+                  ::value == false, "Should fail -- not enough args");
+
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>   // Net
+                       , std::tuple<cv::Rect> >           // Call
+                  ::value == false, "Should fail -- not enough args");
+
+    static_assert(Check< std::tuple<cv::GMat, cv::GMat>             // Net
+                       , std::tuple<cv::Rect, cv::Rect, cv::GMat> > // Call
+                  ::value == false, "Should fail -- too much args");
+}
+} // anonymous namespace
+} // namespace opencv_test