another small bug

[OshriHalimi]

nerf-pytorch/nerf/train_utils.py

Line 176 in a14357d

options,

This function is called also in validation mode, but then, since the variable "mode" is not passed as an argument, the function is called with the default 'train' value.

[Wuao652]

The same problem! Previous implementation will result in a "noisy" output where there will be some "dots" appearing in the scene.