torch/quantization/fake_quantize.py

from __future__ import absolute_import, division, print_function, unicode_literals
import torch
from torch.nn import Module
from .observer import MinMaxObserver, _with_args

class FakeQuantize(Module):
    ''' Simulate the quantize and dequantize operations in training time.
    The output of this module is given by

    x_out = (clamp(round(x/scale + zero_point), quant_min, quant_max)-zero_point)*scale



    * :attr:`scale` defines the scale factor used for quantization.

    * :attr:`zero_point` specifies the quantized value to which 0 in floating point maps to

    * :attr:`quant_min` specifies the minimum allowable quantized value.

    * :attr:`quant_max` specifies the maximum allowable quantized value.

    * :attr:`fake_quant_enable` controls the application of fake quantization on tensors, note that
      statistics can still be updated.

    * :attr:`observer_enable` controls statistics collection on tensors

    * :attr:`dtype` specifies the quantized dtype that is being emulated with fake-quantization,
     allowable values are torch.qint8 and torch.quint8. The values of quant_min and quant_max should
     be chosen to be consistent with the dtype


    Args:
        observer (module): Module for observing statistics on input tensors and calculating
        scale and zero-point.
        quant_min (int): The minimum allowable quantized value.
        quant_max (int): The maximum allowable quantized value.
        observer_kwargs (optional): Arguments for the observer module

    Attributes:
        observer (Module): User provided module that collects statistics on the input tensor and
                           provides a method to calculate scale and zero-point.

    """
    Args:
        `observer`: Observer module that records stats of input tensor
        `quant_min`: Tensors are fake-quantized corresponding to the
        `quant_max`: A function that calculates quantization parameters
        given the stats
        `observer_kwargs`
    '''
    def __init__(self, observer=MinMaxObserver, quant_min=0, quant_max=255, **observer_kwargs):
        super(FakeQuantize, self).__init__()
        assert quant_min <= quant_max, \
            'quant_min must be less than or equal to quant_max'
        self.quant_min = quant_min
        self.quant_max = quant_max
        self.fake_quant_enabled = True
        self.observer_enabled = True
        self.observer = observer(**observer_kwargs)
        assert torch.iinfo(self.observer.dtype).min <= quant_min, 'quant_min out of bound'
        assert quant_max <= torch.iinfo(self.observer.dtype).max, 'quant_max out of bound'
        self.scale = None
        self.zero_point = None
        self.dtype = self.observer.dtype
        self.qscheme = self.observer.qscheme

    def enable_fake_quant(self, enabled=True):
        self.fake_quant_enabled = enabled
        return self

    def disable_fake_quant(self):
        return self.enable_fake_quant(False)

    def calculate_qparams(self):
        return self.observer.calculate_qparams()

    def forward(self, X):
        if self.observer_enabled:
            self.observer(X)
            scale, zero_point = self.calculate_qparams()
            self.scale, self.zero_point = float(scale), int(zero_point)
        if self.fake_quant_enabled:
            X = torch.fake_quantize_per_tensor_affine(X, self.scale, self.zero_point, self.quant_min, self.quant_max)
        return X

    with_args = classmethod(_with_args)

    def extra_repr(self):
        return 'fake_quant_enabled={}, observer_enabled={},\
            scale={}, zero_point={}'.format(
            self.fake_quant_enabled, self.observer_enabled,
            self.scale, self.zero_point)

    def _save_to_state_dict(self, destination, prefix, keep_vars):
        # We cannot currently register scalar values as buffers, so need to manually
        # specify serialization here.
        super(FakeQuantize, self)._save_to_state_dict(destination, prefix, keep_vars)
        destination[prefix + 'scale'] = self.scale
        destination[prefix + 'zero_point'] = self.zero_point

    def _load_from_state_dict(self, state_dict, prefix, local_metadata, strict,
                              missing_keys, unexpected_keys, error_msgs):

        self.scale = state_dict.pop(prefix + 'scale')
        self.zero_point = state_dict.pop(prefix + 'zero_point')
        super(FakeQuantize, self)._load_from_state_dict(state_dict, prefix, local_metadata, False,
                                                        missing_keys, unexpected_keys, error_msgs)

default_fake_quant = FakeQuantize.with_args(observer=MinMaxObserver, quant_min=0, quant_max=255,
                                            dtype=torch.quint8, qscheme=torch.per_tensor_affine, reduce_range=True)
default_weight_fake_quant = FakeQuantize.with_args(observer=MinMaxObserver, quant_min=-128, quant_max=127,
                                                   dtype=torch.qint8, qscheme=torch.per_tensor_symmetric, reduce_range=False)

def disable_fake_quant(mod):
    if type(mod) == FakeQuantize:
        mod.disable_fake_quant()

def enable_fake_quant(mod):
    if type(mod) == FakeQuantize:
        mod.enable_fake_quant()

def disable_observer(mod):
    if type(mod) == FakeQuantize:
        mod.disable_observer()

def enable_observer(mod):
    if type(mod) == FakeQuantize:
        mod.disable_observer()