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- Faster rendering function via nvidia-cub, shipped with cuda >= 11.0 (Require >=11.6 for out use). ~10% speedup
- Expose transmittance computation.

This reverts commit 301b4dfa.

* support alpha for both marching and rendering; relax pytorch requirement

* bump version

* Support python 3.7

* Add dependency

* update docs for ray gradients

* update pdf

* zeros -> emtpy in cuda

* disable cuda hint if is been built

* update examples with better perf

* bump version to 0.2.0

* update readme

* update index and readm

* clean up doc

* add distortion loss

* update init

* bump version