The value of `self.cascade` becomes 0 when `bound <= 0.5`, causing errors

[Shenhan Qian]

A remedy could be modifying

torch-ngp/nerf/renderer.py

Line 73 in b6e0804

 self.cascade = 1 + math.ceil(math.log2(bound)) 

to

self.cascade = max(1 + math.ceil(math.log2(bound)), 1)