Omit _check_valid_rotation_matrix by default

Summary:
According to the profiler trace D40326775, _check_valid_rotation_matrix is slow because of aten::all_close operation and _safe_det_3x3 bottlenecks. Disable the check by default unless environment variable PYTORCH3D_CHECK_ROTATION_MATRICES is set to 1.

Comparison after applying the change:
```
Profiling/Function    get_world_to_view (ms)   Transform_points(ms)    specular(ms)
before                12.751                    18.577                  21.384
after                 4.432 (34.7%)             9.248 (49.8%)           11.507 (53.8%)
```

Profiling trace:
https://pxl.cl/2h687
More details in https://docs.google.com/document/d/1kfhEQfpeQToikr5OH9ZssM39CskxWoJ2p8DO5-t6eWk/edit?usp=sharing

Reviewed By: kjchalup

Differential Revision: D40442503

fbshipit-source-id: 954b58de47de235c9d93af441643c22868b547d0

pytorch3d/transforms/transform3d.py

@@ -5,6 +5,7 @@
 # LICENSE file in the root directory of this source tree.
 
 import math
+import os
 import warnings
 from typing import List, Optional, Union
 
@@ -636,7 +637,10 @@ class Rotate(Transform3d):
             msg = "R must have shape (3, 3) or (N, 3, 3); got %s"
             raise ValueError(msg % repr(R.shape))
         R = R.to(device=device_, dtype=dtype)
-        _check_valid_rotation_matrix(R, tol=orthogonal_tol)
+        if os.environ.get("PYTORCH3D_CHECK_ROTATION_MATRICES", "0") == "1":
+            # Note: aten::all_close in the check is computationally slow, so we
+            # only run the check when PYTORCH3D_CHECK_ROTATION_MATRICES is on.
+            _check_valid_rotation_matrix(R, tol=orthogonal_tol)
         N = R.shape[0]
         mat = torch.eye(4, dtype=dtype, device=device_)
         mat = mat.view(1, 4, 4).repeat(N, 1, 1)