Kwimage

Fixed:
* fixed issue where non-max-suppression couldn't find a default implementation.

Changed:
* Faster color casting

Fixed:
* Misc coco fixes.

Added
* Tentative `Color._forimage` method

Changed
* Simpler demo data file names.
* The kwimage.struct `draw_on` methods now operate in-place (more often) when possible.

Fixed
* Fixed color in kwimage.struct `draw_on` methods.
* Detections concatenate now works when segmentations are populated.

Added
* `imresize` now accepts `letterbox` flag.
* add `numpy` and `tensor` methods to `Mask`

Changed
* `ensure_alpha_channel` now accepts `dtype` and `copy` kwargs.
* `Mask.draw_on` will now draw on images larger or smaller than the mask itself


Fixed
* Fixed Boxes.draw
* `Boxes.draw_on` now works correctly on uint8 and float32 images with 1, 3, or 4 channels
* `Masks.draw_on` now works correctly uint8 or float32 images
* Fixed error in `draw_clf_on_images`
* Fixed scale error in `Detections.random` when `segmentations=True`

Added
* "torchvision" nms mode.
* Ported pure-image drawing functions from kwplot. These are `draw_boxes_on_image`, `draw_clf_on_image`,
`draw_text_on_image`, `make_heatmask`, `make_orimask`,
`make_vector_field`.
* Ported Color from kwplot


Fixed
* Using the new (correct) torch defaults for `align_corners` in `warp_tensor` when possible.
* Fixed bug in "numpy" nms mode.

Changed
* reworked nms auto mode
* nms impl=`cpu` / `gpu` / `py` are now deprecated for `cython_cpu` / `cython_gpu` / `numpy` instead.