bugfix kv cache quantization with ignored layers

src/llmcompressor/modifiers/quantization/cache.py

@@ -87,8 +87,12 @@ def update(
                 v_observer_name, quantization_args=self.quantization_args
             )
 
-            self.k_observers.append(k_observer)
-            self.v_observers.append(v_observer)
+            # NOTE: User may ignore some layers in configuration,
+            # meaning len(self.k_observers) <= layer_idx-1
+            # Must account for that case by padding list so that
+            # index of lists corresponds to layer_idx
+            _pad_and_append_at_idx_(self.k_observers, layer_idx, k_observer)
+            _pad_and_append_at_idx_(self.v_observers, layer_idx, v_observer)
 
         q_key_states = self._quantize(
             key_states.contiguous(), KVCacheScaleType.KEY, layer_idx
@@ -151,12 +155,8 @@ def _quantize(self, tensor, kv_type, layer_idx):
             zps = self.v_zps
 
         scale, zp = observer(tensor)
-        if len(scales) <= layer_idx:
-            scales.append(scale)
-            zps.append(zp)
-        else:
-            scales[layer_idx] = scale
-            zps[layer_idx] = scale
+        _pad_and_append_at_idx_(scales, layer_idx, scale)
+        _pad_and_append_at_idx_(zps, layer_idx, zp)
 
         q_tensor = quantize(
             x=tensor,
@@ -185,3 +185,24 @@ def _dequantize(self, qtensor, kv_type, layer_idx):
             args=self.quantization_args,
         )
         return qdq_tensor
+
+
+# NOTE: Using _ suffix to denote l is modified in place
+def _pad_and_append_at_idx_(lst: List, idx: int, val: Any) -> list:
+    """
+    Append value val to list lst at index idx, right padding if necessary
+    Needed because user may ignore some layers in configuration, meaning
+    len(lst) <= idx-1
+
+    >>> _pad_and_append_at_idx_([0,1,2], 5, 5)
+    [0, 1, 2, None, None, 5]
+    >>> _pad_and_append_at_idx_([0,1,2], 3, 8)
+    [0, 1, 2, 8]
+    >>> _pad_and_append_at_idx_([0,1,2], 1, 5)
+    [0, 5, 2]
+    """
+    num_to_pad = idx - len(lst) + 1
+    if num_to_pad > 0:
+        lst += [None] * num_to_pad
+    lst[idx] = val
+    return lst