Bug in depth backward

[Yihua]

There exists a bug of lacking a parentheses here, referring to the issue.
The backward of depth estimation may encounter CUDA error based on the current codes.

[kiui]

@yihua7 Thanks for spotting this! I have fixed it in d210917.

[Yihua]

Sure :)
By the way, I really appreciate your excellent projects. Based on torch-ngp I developed my SIGGRAPH paper NeRF-Texture. I have acknowledged it on the website.
Looking forward to your future works and learning more from you :)

[kiui]

Nice work! Thanks!