dnn: fix outputs handling with OpenVINO backend

[alalek]

Failed nightly builds:

[  FAILED  ] 5 tests, listed below:
[  FAILED  ] Test_ONNX_layers.Convolution_variable_weight/0, where GetParam() = NGRAPH/CPU
[  FAILED  ] Test_ONNX_layers.Conv1d/0, where GetParam() = NGRAPH/CPU
[  FAILED  ] Test_ONNX_layers.Conv1d_bias/0, where GetParam() = NGRAPH/CPU
[  FAILED  ] Test_ONNX_layers.Conv1d_variable_weight/0, where GetParam() = NGRAPH/CPU
[  FAILED  ] Test_ONNX_layers.PoolConv1d/0, where GetParam() = NGRAPH/CPU

Validate:

• 4.x branch

force_builders=Custom,Custom Win,Custom Mac

build_image:Custom=ubuntu-openvino-2021.4.2:20.04
Xbuild_image:Custom=ubuntu-openvino-2022.1.0.dev20220131:20.04
build_image:Custom Win=openvino-2021.4.2
build_image:Custom Mac=openvino-2021.4.2

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

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

buildworker:Custom Win=windows-3
test_bigdata:Custom Win=1
test_filter:Custom Win=*
test_opencl:Custom Win=ON
build_contrib:Custom Win=OFF

allow_multiple_commits=1

[alalek]

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