Allow alpha transparency training for blender datasets input (#3088)

Allow alpha transparency training for rgba input

Co-authored-by: Justin Kerr <justin.g.kerr@gmail.com>

nerfstudio/data/dataparsers/blender_dataparser.py

@@ -41,8 +41,9 @@ class BlenderDataParserConfig(DataParserConfig):
     """Directory specifying location of data."""
     scale_factor: float = 1.0
     """How much to scale the camera origins by."""
-    alpha_color: str = "white"
-    """alpha color of background"""
+    alpha_color: Optional[str] = "white"
+    """alpha color of background, when set to None, InputDataset that consumes DataparserOutputs will not attempt 
+    to blend with alpha_colors using image's alpha channel data. Thus rgba image will be directly used in training. """
     ply_path: Optional[Path] = None
     """Path to PLY file to load 3D points from, defined relative to the dataset directory. This is helpful for
     Gaussian splatting and generally unused otherwise. If `None`, points are initialized randomly."""