Pull Request Readiness Checklist

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

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to 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

Bugs overview

1. Outputs map are filled only for LoadNetwork case.
   These maps: https://github.com/opencv/opencv/blob/master/modules/gapi/src/backends/ie/giebackend.cpp#L221-L222 are filled only in case LoadNetwork: https://github.com/opencv/opencv/blob/master/modules/gapi/src/backends/ie/giebackend.cpp#L241-L242, but are used by both of them:
   https://github.com/opencv/opencv/blob/master/modules/gapi/src/backends/ie/giebackend.cpp#L953
2. cv::gapi::ie::Params::pluginConfig(IEConfig) set config globally to IE::Core, but should be set locally within ExecutableNetwork: https://github.com/opencv/opencv/blob/master/modules/gapi/src/backends/ie/giebackend.cpp#L244-L245
3. In case ImportNetwork preprocessing should be passed as ExecutableNetwork::SetBlob argument, now preprocessing for NV12 color format is missed.

Build configuration

force_builders=Custom,Custom Win,Custom Mac
build_gapi_standalone:Linux x64=ade-0.1.1f
build_gapi_standalone:Win64=ade-0.1.1f
build_gapi_standalone:Mac=ade-0.1.1f
build_gapi_standalone:Linux x64 Debug=ade-0.1.1f

Xbuild_image:Custom=centos:7
Xbuildworker:Custom=linux-1
build_gapi_standalone:Custom=ade-0.1.1f

build_image:Custom=ubuntu-openvino-2021.4.1:20.04
build_image:Custom Win=openvino-2021.4.1
build_image:Custom Mac=openvino-2021.4.1

test_modules:Custom=gapi,python2,python3,java
test_modules:Custom Win=gapi,python2,python3,java
test_modules:Custom Mac=gapi,python2,python3,java

buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*