Ndsampler

Added
* Added `finalize` argument to `CocoSampler.load_sample` which allows the
sampler to return the delayed image instead of the finalized one.

* The target dictionary for `load_sample` now respects `image_id`, `image_ids`,
and `video_id`.

Fix

* When using the 3d `load_sample` backend with a single image, allow fallback
on image width/height

* Issue when using `visible_thresh`, and added corresponding test.

Removed
* Drop Python 3.6 and 3.7 support

Changed
* Speed improvements to delayed image loading

Added

* New method `load_target_annots` that just finds overlapping annotations

Added
* `load_sample` now takes an `annot_ids` argument to skip the spatial lookup
and use something precomputed.

* Can now specify `optimize=False` in `load_sample` target dictionary to
prevent delayed image from doing any optimization.