Investigate onnx.compose as a potential MergeONNXModels replacement

[maltanar]

Details

For merging models https://onnx.ai/onnx/api/compose.html also exists and we should consider making the MergeONNXModels transform in QONNX a wrapper for onnx.compose instead, provided that we get the same functionality.

New behavior

MergeONNXModels behaves as before, but internally calls onnx.compose to do the model merging. It may still be necessary to copy and merge other fields we make use of in QONNX, such as tensor annotations.

Motivation

This would mean maintaining less code as part of QONNX which would be desirable.

Parts of QONNX being affected

The MergeONNXModels transformation.