[Fix] Convert rt_mat to torch.Tensor in Coord transform for compatibility (#709)

* converted rt_mat from ndarray to tensor to make it further compatible

* changed if condition of rt_mat to be consistent with code

mmdet3d/core/bbox/structures/coord_3d_mode.py

@@ -243,6 +243,8 @@ class Coord3DMode(IntEnum):
                 f'Conversion from Coord3DMode {src} to {dst} '
                 'is not supported yet')
 
+        if not isinstance(rt_mat, torch.Tensor):
+            rt_mat = arr.new_tensor(rt_mat)
         if rt_mat.size(1) == 4:
             extended_xyz = torch.cat(
                 [arr[:, :3], arr.new_ones(arr.size(0), 1)], dim=-1)