[Quantization] fix parameter assign problem of weight (#4138)

nni/algorithms/compression/pytorch/quantization/quantizers.py

@@ -253,7 +253,7 @@ class ObserverQuantizer(Quantizer):
                                             module.weight_qmin,
                                             module.weight_qmax)
                 delattr(module, 'weight')
-                module.register_parameter('weight', torch.nn.Parameter(quantized_weight))
+                module.register_buffer('weight', quantized_weight)
             if "input" in config.get("quant_types", []):
                 scale, zero_point = self.calculate_qparams(layer.name, 'input')
                 module.register_buffer('input_scale', scale.to(self.device))
@@ -301,6 +301,14 @@ class ObserverQuantizer(Quantizer):
                 calibration_config[name]['tracked_min_weight'] = -val
                 calibration_config[name]['tracked_qmin_weight'] = -127
                 calibration_config[name]['tracked_qmax_weight'] = 127
+                weight = module.weight
+                quantized_weight = self._quantize(weight,
+                                            module.weight_scale,
+                                            module.weight_zero_point,
+                                            module.weight_qmin,
+                                            module.weight_qmax)
+                delattr(module, 'weight')
+                module.register_parameter('weight', torch.nn.Parameter(quantized_weight))
             # refactor these magic numbers when customizations of dtype and qscheme are ready.
             if hasattr(module, 'input_scale'):
                 calibration_config[name]['input_bits'] = 8