Export Cronformer with support for dynamic sequence length in MHA export

I applied a fix similar to https://github.com/pytorch/pytorch/pull/111800/files in PyTorch's codebase to get this export.

cronformer.onnx

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:2476b00a5d4b07dd220508244152dd39343569287668bcbceb7f9b65b35857da
-size 417548036
+oid sha256:e4dd00581fc7c4b9f25064138d6e122a38e02173c65c7271d98982300ca59670
+size 417548043