[model_free_ptq] Add warning/error for invalid partner tensors in build_inverse_weights_map (#2546)

## Purpose
Follow-up to #2498 addressing review comments from @kylesayrs.

## Changes
- `partner_shard is None`: log warning instead of silent skip —
indicates
unexpected model architecture where expected partner tensor doesn't
exist
- `partner_resolved is None`: raise ValueError instead of silent skip —
indicates corrupt or incomplete checkpoint and should surface as an
error

## Notes
These cases were flagged as defensive guards that either shouldn't exist
or should error loudly. The warning approach for partner_shard=None
handles
the edge case of non-standard model architectures gracefully while still
surfacing the issue.

## Test
Unit tests passed 35/35

Signed-off-by: David Zheng <dqzheng1996@gmail.com>
Signed-off-by: root <root@bolt-6jxv69gfv8-tqazfcpmhh.bolt-pods.turi-bolt.svc.kube.us-east-1d.k8s.cloud.apple.com>
Co-authored-by: root <root@bolt-6jxv69gfv8-tqazfcpmhh.bolt-pods.turi-bolt.svc.kube.us-east-1d.k8s.cloud.apple.com>
Co-authored-by: Brian Dellabetta <brian-dellabetta@users.noreply.github.com>

Author: 3 people

src/llmcompressor/entrypoints/model_free/microscale.py

@@ -2,6 +2,7 @@
 from collections import defaultdict
 
 from compressed_tensors.quantization import QuantizationScheme, QuantizationStrategy
+from loguru import logger
 
 from llmcompressor.entrypoints.model_free.helpers import (
     MatchedNamesSet,
@@ -121,12 +122,23 @@ def build_microscale_inverse_weights_map(
                 partner_name = partner_template.format(**match.groupdict())
 
                 partner_shard = weight_map.get(partner_name)
-                if partner_shard is None or partner_shard == shard_name:
-                    continue  # same shard or not found
+                if partner_shard is None:
+                    logger.warning(
+                        f"Expected partner tensor {partner_name!r} not found "
+                        f"in weight_map — skipping. This may indicate an "
+                        f"unexpected model architecture."
+                    )
+                    continue
+                if partner_shard == shard_name:
+                    continue  # partner is in the same shard
 
                 partner_resolved = model_files.get(partner_shard)
                 if partner_resolved is None:
-                    continue
+                    raise ValueError(
+                        f"Partner shard {partner_shard!r} for tensor "
+                        f"{partner_name!r} not found in model_files. "
+                        "This indicates a corrupt or incomplete checkpoint."
+                    )
 
                 if partner_name not in inverse_weights_map[partner_resolved]:
                     inverse_weights_map[partner_resolved].append(partner_name)