get rid of gsplat legacy dep (#3386)

Co-authored-by: Ruilong Li <[email protected]>

nerfstudio/models/splatfacto.py

@@ -25,7 +25,6 @@
 
 import numpy as np
 import torch
-from gsplat.cuda_legacy._torch_impl import quat_to_rotmat
 
 try:
     from gsplat.rendering import rasterization
@@ -47,6 +46,26 @@
 from nerfstudio.utils.rich_utils import CONSOLE
 
 
+def quat_to_rotmat(quat):
+    assert quat.shape[-1] == 4, quat.shape
+    w, x, y, z = torch.unbind(quat, dim=-1)
+    mat = torch.stack(
+        [
+            1 - 2 * (y**2 + z**2),
+            2 * (x * y - w * z),
+            2 * (x * z + w * y),
+            2 * (x * y + w * z),
+            1 - 2 * (x**2 + z**2),
+            2 * (y * z - w * x),
+            2 * (x * z - w * y),
+            2 * (y * z + w * x),
+            1 - 2 * (x**2 + y**2),
+        ],
+        dim=-1,
+    )
+    return mat.reshape(quat.shape[:-1] + (3, 3))
+
+
 def random_quat_tensor(N):
     """
     Defines a random quaternion tensor of shape (N, 4)