[Docs] Add source docstrings for QDP constructor and alias API entries

[viiccwen]

What

The QDP Python API source is missing docstrings for a few public API entries:

• qdp/qdp-python/qumat_qdp/backend.py: QdpEngine.__init__
• qdp/qdp-python/qumat_qdp/loader.py: QuantumDataLoader.__init__
• qdp/qdp-python/qumat_qdp/tensor.py: QuantumTensor

These entries are part of the public Python API surface, but the source does not describe the constructor arguments or the compatibility alias directly where API documentation tools can discover them.

Why

The project generates API reference documentation from Python source. Missing source docstrings make generated API docs less useful and force users to infer constructor behavior from class-level prose or implementation details.

Keeping these details in source docstrings also avoids relying on generated-doc post-processing or long documentation config rules for small public API gaps.

How

Add concise source-level documentation for the missing QDP API entries:

• Document QdpEngine.__init__ parameters and unsupported-backend error.
• Document QuantumDataLoader.__init__ default batching settings and validation behavior.
• Add a doc comment for the QuantumTensor = QdpTensor backward-compatible alias.