colormaps: add "gray" (#2058)

* colormaps: remove useless "pca" choice, add "gray"

- The "pca" choice didn't work when applied to a single-channel image (it crashed the viewer), and it's applied by default to any image of more than 3 channels, where the colormap choice is ignore anyway.
- Add the trivial "gray" colormap, useful to eg extract accumulation.

* colormaps.py: re-add "pca"

* control_panel: don't show the "pca" option either

---------

Co-authored-by: Frédéric Devernay <f-dy@users.noreply.github.com>

nerfstudio/utils/colormaps.py

@@ -24,7 +24,7 @@ from torch import Tensor
 
 from nerfstudio.utils import colors
 
-Colormaps = Literal["default", "turbo", "viridis", "magma", "inferno", "cividis", "pca"]
+Colormaps = Literal["default", "turbo", "viridis", "magma", "inferno", "cividis", "gray", "pca"]
 
 
 @dataclass(frozen=True)
@@ -104,6 +104,8 @@ def apply_float_colormap(image: Float[Tensor, "*bs 1"], colormap: Colormaps = "v
         colormap = "turbo"
 
     image = torch.nan_to_num(image, 0)
+    if colormap == "gray":
+        return image.repeat(1, 1, 3)
     image_long = (image * 255).long()
     image_long_min = torch.min(image_long)
     image_long_max = torch.max(image_long)