Forward Catalyst subprocess diagnostics to Python

[sengthai]

Context:
Successful Catalyst subprocess calls capture both output streams but previously returned only stdout, silently discarding warnings and other diagnostics written to stderr. This prevented warnings from appearing in PennyLane workflows such as qp.specs.

We also has verbose but this will output like system log which are not necessary for end users.

Here, we forward captured Catalyst stderr while preserving stdout as normal. When Python’s stderr is attached to a terminal, pass --color to Catalyst so LLVM diagnostics retain their colors. Note that to make this work PL needs to update to pass the stderr_return=True.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.92%. Comparing base (c67c55b) to head (d64ec66).

Files with missing lines	Patch %	Lines
frontend/catalyst/compiler.py	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3080      +/-   ##
==========================================
- Coverage   96.92%   96.92%   -0.01%     
==========================================
  Files         166      166              
  Lines       19133    19138       +5     
  Branches     1815     1817       +2     
==========================================
+ Hits        18545    18549       +4     
  Misses        429      429              
- Partials      159      160       +1     

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[mehrdad2m]

Thanks @sengthai. I noticed this is very similar to what

catalyst/frontend/catalyst/python_interface/inspection/xdsl_conversion.py

Lines 89 to 99 in 0d5db5f

	def _catalyst(*args, stdin=None, stderr_return=False):
	"""Raw interface to catalyst"""
	cmd = _get_catalyst_cli_cmd(*args, stdin=stdin)
	try:
	result = subprocess.run(cmd, input=stdin, check=True, capture_output=True, text=True)
	if stderr_return:
	return result.stdout, result.stderr
	return result.stdout # pragma: no cover
	except subprocess.CalledProcessError as e: # pragma: no cover
	raise CompileError(f"catalyst failed with error code {e.returncode}: {e.stderr}") from e

does for the inspection module in python_interface. Do you think it make sense to unify them and follow the same approach of making it optional?

[sengthai]

@mehrdad2m Thanks and good to know! I made a change and mirror the xdsl's approach b7a204f. And I just realized that qjit has verbose flag to print those diagnostic from Catalyst. So, here I enable this in qp.specs.
PennyLaneAI/pennylane#9938