handle list of names

[juliansjungwirth]

in tf 2.2, 2.3 one can encounter that gathering the tensors names, they sometimes are lists of tensors. this commit handles this. related issue: https://github.com/onnx/keras-onnx/issues/602

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Julian seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}