[code-reflection] RFR: ONNX fixes

[Adam Sotona]

This patch includes:
- `OnnxTransformer` fix of returning in-code constructed list of tensors
- `README.md` fix of the examples execution
- workaround to avoid CNFE when the `ReleaseEnv` class is attempted to load in the above shutdown hook from already closed classloader

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Commit messages:
 - workaround to avoid CNFE when the ReleaseEnv class is attempted to load in the above shutdown hook from already closed classloader
 - fixed ONNX readme
 - OnnxTransformer fix

Changes: https://git.openjdk.org/babylon/pull/565/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=565&range=00
  Stats: 10 lines in 3 files changed: 6 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/babylon/pull/565.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/565/head:pull/565

PR: https://git.openjdk.org/babylon/pull/565