add validation check on format (#424)

Author: dsikka

src/compressed_tensors/quantization/quant_scheme.py

@@ -57,6 +57,7 @@ def validate_model_after(model: "QuantizationScheme") -> "QuantizationScheme":
         inputs = model.input_activations
         outputs = model.output_activations
         weights = model.weights
+        format = model.format
 
         if inputs is not None:
             if inputs.actorder is not None:
@@ -66,6 +67,11 @@ def validate_model_after(model: "QuantizationScheme") -> "QuantizationScheme":
             if outputs.actorder is not None:
                 raise ValueError("Cannot apply actorder to output activations")
 
+        if format == CompressionFormat.mixed_precision.value:
+            raise ValueError(
+                "mixed-precision cannot be set as a format for a QuantizationScheme"
+            )
+
         if (
             inputs
             and weights