fix(quant_api): prevent IndexError in _fqn_to_config_handler when multiple parameter FQNs are None (#4818) - #4901

Summary

Fixes #4818

In _fqn_to_config_handler (torchao/quantization/quant_api.py), when multiple parameter FQNs are set to None in FqnToConfig to exclude them from quantization, the previous implementation called top_level_params.pop(i):

if c is None:
    top_level_params.pop(i)  # BUG: i is the named_parameters index, not current list index

When two or more parameters were set to None, popping reduced the list size, and subsequent pop(i) calls either targeted the wrong element or failed with IndexError: pop index out of range.

Solution

Filter top_level_params via list comprehension to safely exclude parameters matching parameter_fqn without index mutation side-effects:

if c is None:
    top_level_params = [
        entry for entry in top_level_params if entry[3] != parameter_fqn
    ]

Test Plan

• Added unit test test_fqn_to_config_multiple_none_parameters in test/quantization/test_quant_api.py under TestQuantFlow (CPU-compatible).
• Verified test passes:

  pytest test/quantization/test_quant_api.py -k test_fqn_to_config_multiple_none_parameters -v

• Checked linters: ruff check passes with zero errors.