Revert "Update default observer to be MSE (#300)" (#323)

This reverts commit 8916411.

Author: dsikka

src/compressed_tensors/quantization/quant_args.py

@@ -269,8 +269,8 @@ def validate_model_after(model: "QuantizationArgs") -> "QuantizationArgs":
                 observer = None
 
         elif observer is None:
-            # default to mse for non-dynamic cases
-            observer = "mse"
+            # default to minmax for non-dynamic cases
+            observer = "minmax"
 
         # write back modified values
         model.strategy = strategy