fix device error

Summary: When using `sample_farthest_points` with `lengths`, it throws an error because of the device mismatch between `lengths` and `torch.rand(lengths.size())` on GPU.

Reviewed By: bottler

Differential Revision: D82378997

fbshipit-source-id: 8e929256177d543d1dd1249e8488f70e03e4101f

Author: kihyuks

pytorch3d/ops/sample_farthest_points.py

@@ -89,7 +89,9 @@ def sample_farthest_points(
         if constant_length:
             start_idxs = torch.randint(high=P, size=(N,), device=device)
         else:
-            start_idxs = (lengths * torch.rand(lengths.size())).to(torch.int64)
+            start_idxs = (lengths * torch.rand(lengths.size(), device=device)).to(
+                torch.int64
+            )
     else:
         start_idxs = torch.zeros_like(lengths)