Enhancement of PyTorch connector

[edoaltamura]

What should we add?

Given the demand for robust coupling with PyTorch, we propose to enhance the module

qiskit-machine-learning/qiskit_machine_learning/connectors/torch_connector.py

Line 56 in 60f2c2e

class TorchConnector(Module):

in two phases.

Phase 1 will involve no or very minor changes to the user-facing API. The changes will focus on refractoring the backend connector, bug patches, and compatibility robustness.

• Fixing compatibility with PyTorch versions 2.x, possibly in qiskit_machine_learning/optionals.py
• Refactoring _TorchNNFunction and other definitions in torch_connector.py
• Refractoring code that involves

  qiskit-machine-learning/qiskit_machine_learning/connectors/torch_connector.py

  Line 215 in 60f2c2e

  if neural_network.sparse:

  into modular units based on _optionals.HAS_SPARSE.require_now("SparseArray")

Phase 2 will focus on upgrading the inter-functionality with PyTorch and may involve changes in API and UX. These changes are likely to be incrementally introduced after the version 0.8 roll-out, leading up to a stable 1.0.

• Adding version compatibility between Qiskit machine-learning and PyTorch, with relevant deprecation warnings and supported features
• Improving the test coverage
• Improved documentation