First version of GraphBuilder (#2814)

First version of GraphBuilder.

TODO later:
* Integrate schema-based separation of inputs and attributes
* Align behaviors of script-mode and the new trace-mode as much as
possible
  - With respect to conversion of python constants into ONNX tensors
  - With respect to conversion of python values into ONNX attributes
- With respect to support for special kwargs like _domain and _version
(to be added to script mode)
- Consider supporting op.builder and op.builder.opset(...) in script
mode
* Integrate calls to script-mode functions from trace-mode (and
vice-versa)

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Signed-off-by: Ganesan Ramalingam <grama@microsoft.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Justin Chu <justinchuby@users.noreply.github.com>

Author: 3 people