Fix logging trace statement with empty gradient_fn

[mlxd]

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---

Context: The logging implementation for the TRACE level assumes the existence of several functions to report their source when enabled. If the gradient_fn is None, the logger attempts to retrieve the source and fails.

Description of the Change: This adds an existence check for all execute-enabled TRACE level log statements, and reports only if the associated functions exist.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[mlxd]

[sc-47564]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5cc309b) 99.64% compared to head (fdadb59) 99.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4669      +/-   ##
==========================================
- Coverage   99.64%   99.63%   -0.01%     
==========================================
  Files         377      377              
  Lines       34002    33741     -261     
==========================================
- Hits        33881    33619     -262     
- Misses        121      122       +1     

Files	Coverage Δ
pennylane/interfaces/autograd.py	100.00% <ø> (ø)
pennylane/interfaces/execution.py	97.20% <ø> (-0.03%)	⬇️
pennylane/interfaces/jacobian_products.py	100.00% <ø> (ø)
pennylane/interfaces/jax.py	99.00% <ø> (ø)
pennylane/interfaces/tensorflow.py	96.11% <ø> (ø)
pennylane/interfaces/torch.py	96.63% <ø> (ø)
pennylane/qnode.py	100.00% <ø> (ø)

... and 41 files with indirect coverage changes

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[timmysilv]

not sure I totally get what's being handled here. I thought it was the difference when gradient_fn is None, but they seem the same?

>>> def fn1():
...     return 2
>>> fn2 = None
>>> print(isfunction(fn1), isfunction(fn2))
True False
>>> print(callable(fn1), callable(fn2))
True False

[mudit2812]

I think some minimal testing would be good, since this was never caught by CI, otherwise looks good!

[mlxd]

not sure I totally get what's being handled here. I thought it was the difference when gradient_fn is None, but they seem the same?

>>> def fn1():
...     return 2
>>> fn2 = None
>>> print(isfunction(fn1), isfunction(fn2))
True False
>>> print(callable(fn1), callable(fn2))
True False

Did you enable the trace logging level? This config requires explicit edits to the YAML file at

pennylane/pennylane/logging/log_config.toml

Line 86 in 4e71a8f

level = "DEBUG" # Set to TRACE for highest verbosity

I see failures without this, and success with.

[mlxd]

I think some minimal testing would be good, since this was never caught by CI, otherwise looks good!

Testing with the previous logging contribution wasn't easily possible, so this will likely need to be skipped.

[mudit2812]

Thanks!

[timmysilv]

talked offline, looks good 👍