[josephleekl]

Context:
Catalyst now selects transport backends from a registry that backend packages can extend (PennyLaneAI/catalyst#3362). PennyLane still only accepted the transport names it registers itself, so a new transport still needed a PennyLane change.

Description of the Change:

• qp.backline.get_transport keeps its built-in transports (rdma, memcpy, none).
  • Any other name is accepted if a backend in Catalyst's registry implements it, read through catalyst.backline.backend_transports() with a lazy import.
  • Without Catalyst installed, the name is accepted and checked at compile time.
• Removes the qp.Backline.backline alias of placement; nothing reads it any more.
• Updates the executor_options["port"] docstring: the compiler now picks a free port for a local executor ([Backline] Housekeeping before the transport ABI work: HIP switch, demo tests on by default, executor lifecycle catalyst#3355).

Benefits:

• A backend package adds a transport with no PennyLane change.

Possible Drawbacks:

• Breaking: qp.Backline.backline is removed (experimental API). Use placement.

Testing:

• New tests: an unknown transport is rejected; a registry transport is accepted; any name is accepted without Catalyst.
• tests/backline: 227 passed. The Catalyst Backline suite passes against this branch, and qp.Backline(transport="example") resolves to a registered example backend end to end.

Related GitHub Issues:

[sc-134377]

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.43%. Comparing base (1806cda) to head (1ebd539).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pennylane/backline/transports.py	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10353      +/-   ##
==========================================
- Coverage   99.44%   99.43%   -0.01%     
==========================================
  Files         617      617              
  Lines       68037    68043       +6     
==========================================
+ Hits        67656    67657       +1     
- Misses        381      386       +5     

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