fix(global_ptq): make MDBF detection tolerate a missing MDBF quantizer

detect_quantization_method() imported MultipathMDBFLinear
unconditionally, so on an installation without onecomp.quantizer.mdbf
*every* call raised ModuleNotFoundError -- including for plain GPTQ and
DBF models.  On this PR's base branch that broke the existing
TestDetectQuantizationMethod tests (3 failures).

Guard the import and treat MDBF as absent when it cannot be imported,
so GPTQ/DBF detection keeps working without the MDBF quantizer.

Verified both ways: without MDBF the global_ptq unit tests pass
(69 passed, integration tests deselected), and with MDBF present
(merged with feature/mdbf) detection still resolves MultipathMDBFLinear.

Author: fujisawa-yoshihiko

global_ptq/onecomp_globalptq/global_ptq/_core/helpers.py

@@ -81,11 +81,18 @@ def detect_quantization_method(
     """
     from onecomp.quantizer.dbf.dbf_layer import DoubleBinaryLinear
     from onecomp.quantizer.gptq.gptq_layer import GPTQLinear
-    from onecomp.quantizer.mdbf.mdbf_layer import MultipathMDBFLinear
+
+    try:
+        from onecomp.quantizer.mdbf.mdbf_layer import MultipathMDBFLinear
+    except ImportError:
+        # MDBF quantizer is optional; without it only GPTQ/DBF are detectable.
+        MultipathMDBFLinear = None
 
     gptq_modules = find_target_modules(model, GPTQLinear)
     dbf_modules = find_target_modules(model, DoubleBinaryLinear)
-    mdbf_modules = find_target_modules(model, MultipathMDBFLinear)
+    mdbf_modules = (
+        find_target_modules(model, MultipathMDBFLinear) if MultipathMDBFLinear is not None else []
+    )
 
     if gptq_modules and (dbf_modules or mdbf_modules):
         logger.warning(