Fix torch export issue

Summary: X-link: fairinternal/detectron2#604 Pull Request resolved: #5334 Add a check to deal with dynamic shapes so that the model can be exported with `torch.export`. This check prevents the graph break caused by the SymInt by delaying the assertion to runtime. Reviewed By: wat3rBro Differential Revision: D60126415 fbshipit-source-id: a2a75530db523bfdde984b890595e02360d8e07f

Author: jackzhxng

detectron2/export/c10.py

@@ -84,6 +84,11 @@ def set(self, name, value):
  else:
  data_len = len(value)
  if len(self.batch_extra_fields):
+ # If we are tracing with Dynamo, the check here is needed since len(self)
+ # represents the number of bounding boxes detected in the image and thus is
+ # an unbounded SymInt.
+ if torch._utils.is_compiling():
+ torch._check(len(self) == data_len)
  assert (
  len(self) == data_len
  ), "Adding a field of length {} to a Instances of length {}".format(data_len, len(self))