Add interval loss (#92)

Implement interval loss that was proposed in mipnerf-360.

docs/reference/graphs/modules/index.rst

@@ -7,4 +7,5 @@ TODO: High level description of graph modules and how they are used.
 
 .. toctree::
 
-   ray_sampler
+   ray_sampler
+   ray_losses

docs/reference/graphs/modules/ray_losses.rst

@@ -0,0 +1,8 @@
+.. _ray_losses:
+
+Ray Losses
+===================
+
+.. automodule:: pyrad.graphs.modules.ray_losses
+   :members:
+   :show-inheritance: