Add torch._check hints for torch.export

Summary:

Fixes #5347

This allows for torch.export.export ImageList.from_tensors.

Differential Revision: D74835454

Author: yushangdi

detectron2/structures/image_list.py

@@ -8,6 +8,19 @@
 from detectron2.layers.wrappers import move_device_like, shapes_to_tensor
 
 
+try:
+    try:
+        from torch.compiler import is_compiling
+
+    except Exception:
+        # torch.compiler.is_compiling is not available in torch 1.10
+        from torch._dynamo import is_compiling as is_compiling
+
+except Exception:
+
+    def is_compiling() -> bool:  # type: ignore[misc]
+        return False
+
 class ImageList:
     """
     Structure that holds a list of images (of possibly
@@ -111,7 +124,12 @@ def from_tensors(
             # This seems slightly (2%) faster.
             # TODO: check whether it's faster for multiple images as well
             image_size = image_sizes[0]
-            padding_size = [0, max_size[-1] - image_size[1], 0, max_size[-2] - image_size[0]]
+            u0 = max_size[-1] - image_size[1]
+            u1 = max_size[-2] - image_size[0]
+            padding_size = [0, u0, 0, u1]
+            if is_compiling():
+                torch._check(u0.item() >= 0)
+                torch._check(u1.item() >= 0)
             batched_imgs = F.pad(tensors[0], padding_size, value=pad_value).unsqueeze_(0)
         else:
             # max_size can be a tensor in tracing mode, therefore convert to list