Remove error message assertion (pytorch#480)

Depending on how PyTorch is compiled, the source code for DataLoader
might not be fully available which can cause a spurious error in
test_dataloader.py

Author: colesbury

test/test_dataloader.py

@@ -78,8 +78,6 @@ def _test_error(self, loader):
             try:
                 it.next()
             except NotImplementedError:
-                msg = "".join(traceback.format_exception(*sys.exc_info()))
-                self.assertTrue("collate_fn" in msg)
                 errors += 1
             except StopIteration:
                 self.assertEqual(errors,