[code-reflection] RFR: Onnx subgraphs - work in progress

[Adam Sotona]

Onnx subgraphs - work in progress

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Commit messages:
 - implemented Onnx Loop test
 - OnnxProtoBuilder refactoring
 - reduced use of ByteBuffers and Tensor API refactoring
 - Merge branch 'code-reflection' into onnx-runtime-cont6
 - Onnx subgraphs - work in progress
 - Onnx subgraphs - work in progress

Changes: https://git.openjdk.org/babylon/pull/326/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=326&range=00
  Stats: 184 lines in 8 files changed: 92 ins; 38 del; 54 mod
  Patch: https://git.openjdk.org/babylon/pull/326.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/326/head:pull/326

PR: https://git.openjdk.org/babylon/pull/326