GitLab

* Adding output channels implementation for pngs.

* Adding tests for png.

* Adding channels in the API and documentation.

* Fixing formatting.

* Refactoring test_image.py to remove huge grace_hopper_517x606.pth file from assets and reduce duplicate code. Moving jpeg assets used by encode and write unit-tests on their separate folders.

* Adding output channels implementation for jpegs. Fix asset locations.

* Add tests for JPEG, adding the channels in the API and documentation and adding checks for inputs.

* Changing folder for unit-test.

* Fixing windows flakiness, removing duplicate test.

* Replacing components to channels.

* Adding reference for supporting CMYK.

* Minor changes: num_components to output_components, adding comments, fixing variable name etc.

* Reverting output_components to num_components.

* Replacing decoding with generic method on tests.

* Palette converted to Gray.

- Replace deprecated string.h with cstring.
- Switch to nullptr.

* Add decode_image op

* Fix lint

* More lint

* Add C10_EXPORT