add quantization backward support (#1854)

examples/model_compress/QAT_torch_quantizer.py

@@ -35,7 +35,6 @@ def train(model, quantizer, device, train_loader, optimizer):
         loss = F.nll_loss(output, target)
         loss.backward()
         optimizer.step()
-        quantizer.step()
         if batch_idx % 100 == 0:
             print('{:2.0f}%  Loss {}'.format(100 * batch_idx / len(train_loader), loss.item()))
 

src/sdk/pynni/nni/compression/torch/builtin_quantizers.py

@@ -100,7 +100,7 @@ def get_bits_length(config, quant_type):
 
 
 class QAT_Quantizer(Quantizer):
-    """Quantizer using the DoReFa scheme, as defined in:
+    """Quantizer defined in:
     Quantization and Training of Neural Networks for Efficient Integer-Arithmetic-Only Inference
     http://openaccess.thecvf.com/content_cvpr_2018/papers/Jacob_Quantization_and_Training_CVPR_2018_paper.pdf
     """
@@ -227,20 +227,17 @@ class DoReFaQuantizer(Quantizer):
     (https://arxiv.org/abs/1606.06160)
     """
     def __init__(self, model, config_list):
-        """
-        config_list: supported keys:
-            - q_bits
-        """
         super().__init__(model, config_list)
 
     def quantize_weight(self, weight, config, **kwargs):
+        weight_bits = get_bits_length(config, 'weight')
         out = weight.tanh()
         out = out / (2 * out.abs().max()) + 0.5
-        out = self.quantize(out, config['q_bits'])
+        out = self.quantize(out, weight_bits)
         out = 2 * out -1
         return out
 
     def quantize(self, input_ri, q_bits):
         scale = pow(2, q_bits)-1
         output = torch.round(input_ri*scale)/scale
-        return output
+        return output

src/sdk/pynni/nni/compression/torch/compressor.py

@@ -250,6 +250,10 @@ class Quantizer(Compressor):
     Base quantizer for pytorch quantizer
     """
 
+    def __init__(self, model, config_list):
+        super().__init__(model, config_list)
+        self.quant_grad = QuantGrad
+
     def quantize_weight(self, weight, config, op, op_type, op_name):
         """
         quantize should overload this method to quantize weight.
@@ -262,7 +266,7 @@ def quantize_weight(self, weight, config, op, op_type, op_name):
         config : dict
             the configuration for weight quantization
         """
-        raise NotImplementedError("Quantizer must overload quantize_weight()")
+        raise NotImplementedError('Quantizer must overload quantize_weight()')
 
     def quantize_output(self, output, config, op, op_type, op_name):
         """
@@ -276,7 +280,7 @@ def quantize_output(self, output, config, op, op_type, op_name):
         config : dict
             the configuration for output quantization
         """
-        raise NotImplementedError("Quantizer must overload quantize_output()")
+        raise NotImplementedError('Quantizer must overload quantize_output()')
 
     def quantize_input(self, *inputs, config, op, op_type, op_name):
         """
@@ -290,7 +294,7 @@ def quantize_input(self, *inputs, config, op, op_type, op_name):
         config : dict
             the configuration for inputs quantization
         """
-        raise NotImplementedError("Quantizer must overload quantize_input()")
+        raise NotImplementedError('Quantizer must overload quantize_input()')
 
 
     def _instrument_layer(self, layer, config):
@@ -305,62 +309,93 @@ def _instrument_layer(self, layer, config):
             the configuration for quantization
         """
         assert layer._forward is None, 'Each model can only be compressed once'
-        assert "quant_types" in config, 'must provide quant_types in config'
-        assert isinstance(config["quant_types"], list), 'quant_types must be list type'
-        assert "quant_bits" in config, 'must provide quant_bits in config'
-        assert isinstance(config["quant_bits"], int) or isinstance(config["quant_bits"], dict), 'quant_bits must be dict type or int type'
+        assert 'quant_types' in config, 'must provide quant_types in config'
+        assert isinstance(config['quant_types'], list), 'quant_types must be list type'
+        assert 'quant_bits' in config, 'must provide quant_bits in config'
+        assert isinstance(config['quant_bits'], int) or isinstance(config['quant_bits'], dict), 'quant_bits must be dict type or int type'
 
-        if isinstance(config["quant_bits"], dict):
-            for quant_type in config["quant_types"]:
-                assert quant_type in config["quant_bits"], 'bits length for %s must be specified in quant_bits dict' % quant_type
+        if isinstance(config['quant_bits'], dict):
+            for quant_type in config['quant_types']:
+                assert quant_type in config['quant_bits'], 'bits length for %s must be specified in quant_bits dict' % quant_type
 
-        if 'weight' in config["quant_types"]:
+        if 'weight' in config['quant_types']:
             if not _check_weight(layer.module):
                 _logger.warning('Module %s does not have parameter "weight"', layer.name)
+            else:
+                # old_weight is used to store origin weight and weight is used to store quantized weight
+                # the reason why weight is buffer instead of parameter is because in pytorch parameter is used as leaf
+                # if weight is leaf , then old_weight can not be updated.
+                layer.module.register_parameter('old_weight', torch.nn.Parameter(layer.module.weight))
+                delattr(layer.module, 'weight')
+                layer.module.register_buffer('weight', layer.module.old_weight)
+
         layer._forward = layer.module.forward
 
         def new_forward(*inputs):
-            if 'input' in config["quant_types"]:
-                inputs = straight_through_quantize_input.apply(inputs, self, config, layer)
+            if 'input' in config['quant_types']:
+                inputs = self.quant_grad.apply(inputs, QuantType.QUANT_INPUT, self.quantize_input, config, layer)
 
-            if 'weight' in config["quant_types"] and _check_weight(layer.module):
-                weight = layer.module.weight.data
-                new_weight = self.quantize_weight(weight, config, op=layer.module, op_type=layer.type, op_name=layer.name)
-                layer.module.weight.data = new_weight
+            if 'weight' in config['quant_types'] and _check_weight(layer.module):
+                new_weight = self.quant_grad.apply(layer.module.old_weight, QuantType.QUANT_WEIGHT, self.quantize_weight, config, layer)
+                layer.module.weight = new_weight
                 result = layer._forward(*inputs)
-                layer.module.weight.data = weight
             else:
                 result = layer._forward(*inputs)
 
-            if 'output' in config["quant_types"]:
-                result = straight_through_quantize_output.apply(result, self, config, layer)
+            if 'output' in config['quant_types']:
+                result = self.quant_grad.apply(result, QuantType.QUANT_OUTPUT, self.quantize_output, config, layer)
             return result
 
         layer.module.forward = new_forward
 
+class QuantType:
+    """
+    Enum class for quantization type.
+    """
+    QUANT_INPUT = 0
+    QUANT_WEIGHT = 1
+    QUANT_OUTPUT = 2
 
-class straight_through_quantize_output(torch.autograd.Function):
+class QuantGrad(torch.autograd.Function):
+    """
+    Base class for overriding backward function of quantization operation.
+    """
     @staticmethod
-    def forward(ctx, output, quantizer, config, layer):
-        return quantizer.quantize_output(output, config, op=layer.module, op_type=layer.type, op_name=layer.name)
+    def quant_backward(tensor, grad_output, quant_type):
+        """
+        This method should be overrided by subclass to provide customized backward function,
+        default implementation is Straight-Through Estimator
 
-    @staticmethod
-    def backward(ctx, grad_output):
-        # Straight-through estimator
-        return grad_output, None, None, None
+        Parameters
+        ----------
+        tensor : Tensor
+            input of quantization operation
+        grad_output : Tensor
+            gradient of the output of quantization operation
+        quant_type : QuantType
+            the type of quantization, it can be `QuantType.QUANT_INPUT`, `QuantType.QUANT_WEIGHT`, `QuantType.QUANT_OUTPUT`,
+            you can define different behavior for different types.
 
-class straight_through_quantize_input(torch.autograd.Function):
-    @staticmethod
-    def forward(ctx, inputs, quantizer, config, layer):
-        return quantizer.quantize_input(inputs, config, op=layer.module, op_type=layer.type, op_name=layer.name)
+        Returns
+        -------
+        tensor
+            gradient of the input of quantization operation
+        """
+        return grad_output
 
     @staticmethod
-    def backward(ctx, grad_output):
-        # Straight-through estimator
-        return grad_output, None, None, None
+    def forward(ctx, tensor, quant_type, quant_func, config, layer):
+        ctx.save_for_backward(tensor, torch.Tensor([quant_type]))
+        return quant_func(tensor, config, op=layer.module, op_type=layer.type, op_name=layer.name)
+
+    @classmethod
+    def backward(cls, ctx, grad_output):
+        tensor, quant_type = ctx.saved_variables
+        output = cls.quant_backward(tensor, grad_output, quant_type)
+        return output, None, None, None, None
 
 def _check_weight(module):
     try:
-        return isinstance(module.weight, torch.nn.Parameter) and isinstance(module.weight.data, torch.Tensor)
+        return isinstance(module.weight.data, torch.Tensor)
     except AttributeError:
         return False