Add default depth output when no gaussians visible

[chungmin99]

There should be a default "depth" value returned alongside "rgb", or the renderer may crash if there is no gaussians in the front.

More specifically, L173 and L189 will currently crash in nerfstudio if "depth" is undefined:

nerfstudio/nerfstudio/viewer/render_state_machine.py

Lines 170 to 191 in 52cda21

	if self.viewer.control_panel.layer_depth:
	if isinstance(self.viewer.get_model(), SplatfactoModel):
	# Gaussians render much faster than we can send depth images, so we do some downsampling.
	assert len(outputs["depth"].shape) == 3
	assert outputs["depth"].shape[-1] == 1
	desired_depth_pixels = {"low_move": 128, "low_static": 128, "high": 512}[self.state] ** 2
	current_depth_pixels = outputs["depth"].shape[0] * outputs["depth"].shape[1]
	scale = min(desired_depth_pixels / current_depth_pixels, 1.0)
	outputs["gl_z_buf_depth"] = F.interpolate(
	outputs["depth"].squeeze(dim=-1)[None, None, ...],
	size=(int(outputs["depth"].shape[0] * scale), int(outputs["depth"].shape[1] * scale)),
	mode="bilinear",
	)[0, 0, :, :, None]
	else:
	# Convert to z_depth if depth compositing is enabled.
	R = camera.camera_to_worlds[0, 0:3, 0:3].T
	camera_ray_bundle = camera.generate_rays(camera_indices=0, obb_box=obb)
	pts = camera_ray_bundle.directions * outputs["depth"]
	pts = (R @ (pts.view(-1, 3).T)).T.view(*camera_ray_bundle.directions.shape)
	outputs["gl_z_buf_depth"] = -pts[..., 2:3] # negative z axis is the coordinate convention