* add CPU only mode which can be activated during install

* fixed flake8 errors for too long lines, still have to deal with "import not at top of file"

* reversing changes in MinIoURandomCrop, that not relevant to the CPU_ONLY pull request

* moving the CPU_ONLY checks into deeper parts of the code

* completing previous commit

* using isort for imports sorting

* yapf fix

* followed @xvjiarui suggestions for the pull request

* use mmdet.CPU_ONLY and replace the "--cpu" flag in setup.py with automatic check of cuda

* make setup code cleaner

* back to original implementation of MinIoURandomCrop

* build all extensions with CUDA, if available

* fixed DC

* update doc

* fixed masked_conv2d_ext

* set warning once, update comment

Co-authored-by: Yossi Biton <yossi.biton@alibaba-inc.com>
Co-authored-by: Yossi Biton <yossibit10@gmail.com>