Update ONNX-to-HLO converter to support T75 nets

[mooskagh]

• Dependent computation support
• Helper class for HloShapes instead of suing protos directly
• Ops:
  • [x] GlobalAveragePool
  • [x] Squeeze
  • [x] Split
  • [x] Sigmoid
  • [x] Mul
  • [x] Gather