fix: wrong checkpoints path for DepthAnythingModel when input control is null

[DK-DARKmatter]

When I tried with examples given like:

"control_overrides": {"seg": {"input_control": "path/to/video1_seg.mp4"}, "depth": {"input_control": null}}}

I got following error:

 no input_control provided for depth. generating input control video with DepthAnythingModel
Loading Depth Anything model - depth-anything/Depth-Anything-V2-Small-hf...
OSError: We couldn't connect to 'https://huggingface.co' to load this file, couldn't find it in the cached files and it looks like depth-anything/Depth-Anything-V2-Small-hf is not the path to a directory containing a file named preprocessor_config.json.

The correct relative path should be:

checkpoints/depth-anything/Depth-Anything-V2-Small-hf

if we use script to download all dependencies. The code here forget to make a path join so it does not read from the correct path, which I think has a different behavior from doc.
This is a relative simple fix, please tell me if we have a more elegant way to fix this.
BTW, I think this issue #79 has the same problem as this one. Maybe we could fix that too.