fix max_unpool missing attributes, add default value of keepdims in reducemean/max/sum, add support for keepdims=true in full reduction branch, add new padding type to Pad

Onnx conformance tests

* Add ONNX conformance tests

* dnn(test): add filters for ONNX conformance tests

* add filter lists for OCV backend

* address review comments

* move test_clip_inbounds to all_denylist

* address clip issue

* avoid empty lists
Co-authored-by: NAlexander Alekhin <alexander.a.alekhin@gmail.com>

- no NaN propagation guarantee

When computing:
t1 = (bayer[1] + bayer[bayer_step] + bayer[bayer_step+2] + bayer[bayer_step*2+1])*G2Y;
there is a T (unsigned short or char) multiplied by an int which can overflow.
Then again, it is stored to t1 which is unsigned so the overflow disappears.
Keeping all unsigned is safer.

paper under.

fix cvtColor-error

* fix gray image channel error

* fix gray image channel error

* fix cvtColor error after the video end

* fix cvtColor error after the video end and change next variable

* fix cvtColor error after the video end

* reset next variable

* fix cvtColor error after the video end

* fix cvtColor error after the video end

Fow now, it is possible to define valid rectangle for which some
functions overflow (e.g. br(), ares() ...).
This patch fixes the intersection operator so that it works with
any rectangle.

* Replaced distutils module to sysconfig

* Fixed getting a path to python lib

fix issue 21187: markdown file: gray image does not have three dimensions

fix ceil_mode for Average/MaxPooling

* fix ceil_mode

* add a comment

* Fix integer overflow in cv::Luv2RGBinteger::process.

For LL=49, uu=205, vv=23, we end up with x=7373056 and y=458
which overflows y*x.

* imgproc(test): adjust test parameters to cover SIMD code