剪枝在pytorch中是如何实现的？

Pytorch中剪枝源码可参考：
https://github.com/pytorch/pytorch/blob/master/torch/nn/utils/prune.py
可参考：
pytorch中函数接口：https://runebook.dev/zh-CN/docs/pytorch/-index-#nn

在Pytorch中的剪枝操作一文中，自定义剪枝中提到剪枝操作继承自BasePruningMethod基类 ，并且子类中需要单独实现__init__ 和 compute_mask (mask和参数所执行的逻辑操作)，并指明执行哪种剪枝类型 ( global,structured, 或者 unstructured)，这一讲就来看下，其中是如何实现的。

类图结构

可看到裁剪策略有L1非结构化剪枝、Ln结构化剪枝、随机非结构化剪枝、自定义剪枝等等都是继承自BasePruningMethod，另还有一个非常重要的子类：PruningContainer，为迭代修剪而持有修剪方法序列的容器。跟踪应用修剪方法的顺序,并处理连续修剪调用的组合。对于同一个module使用多个剪枝策略时，pytorch通过PruningContainer来对剪枝策略进行管理。PruningContainer本身也是继承自BasePruningMethod。同时设置前向计算的回调，便于后续训练时调用。

基类 BasePruningMethod

基类BasePruningMethod为一个抽象类，提供了剪枝方法的框架

class BasePruningMethod(ABC):
    r"""需要自己实现compute_mask和apply方法
    """
    _tensor_name: str

    def __init__(self):
        pass

    # 调用apply_mask
    def __call__(self, module, inputs):
        ...

    @abstractmethod
    def compute_mask(self, t, default_mask):
        r"""计算mask tensor，输入tensor t，输出和t相同维度的mask
        """
        pass

    def apply_mask(self, module):
        r"""简单将待剪得parameter 和  mask相乘，输入mask和原始tensor，返回剪之后的tensor.
        """
        ...
        pruned_tensor = mask.to(dtype=orig.dtype) * orig
        return pruned_tensor

    @classmethod
    def apply(cls, module, name, *args, importance_scores=None, **kwargs):
        r"""增加forward pre-hook 可以在forward()时完成original tensor
        和 pruning mask的reparametrization
        """

        def _get_composite_method(cls, module, name, *args, **kwargs):
            old_method = None
            found = 0
            # 一个module只允许一个_forward_pre_hook
            ...
            assert (
                found <= 1
            ), "Avoid adding multiple pruning hooks to the\
                same tensor {} of module {}. Use a PruningContainer.".format(
                name, module
            )

            ...

            # 创建pruning container包含多个pruning method
            # combine `methods` with `old_method`, if `old_method` exists
            ...
                    container = PruningContainer(old_method)
                    # Have the pruning method remember the name of its tensor
                    # setattr(container, '_tensor_name', name)
                    container.add_pruning_method(method)
                    method = container  # rename container --> method
            return method

        method = _get_composite_method(cls, module, name, *args, **kwargs)

        ...

        # 
        # 第一次裁剪，初始化default_mask,将原param tensor移动到一个新参数name + '_orig' 并删除原来 parameter
        if not isinstance(method, PruningContainer):
            # copy `module[name]` to `module[name + '_orig']`
            module.register_parameter(name + "_orig", orig)
            # temporarily delete `module[name]`
            del module._parameters[name]
            default_mask = torch.ones_like(orig)  # temp
        # 不是第一次裁剪
        # If this is not the first time pruning is applied, all of the above
        # has been done before in a previous pruning iteration, so we're good
        # to go
        else:
            default_mask = (
                getattr(module, name + "_mask")
                .detach()
                .clone(memory_format=torch.contiguous_format)
            )

        # Use try/except 避免意外来回滚
        # 计算compute_mask 并register_forward_pre_hook
        try:
            # 依据importance_scores来compute_mask
            mask = method.compute_mask(importance_scores, default_mask=default_mask)
            # 保存 mask to `module[name + '_mask']` 缓存
            module.register_buffer(name + "_mask", mask)
            # 以及pruned tensor 存到 `module[name]` 状态
            setattr(module, name, method.apply_mask(module))
            # 通过hook，register_forward_pre_hook，关联module的pruning到的forward()中，这样推理时也可以做reparam
            module.register_forward_pre_hook(method)

        except Exception as e:
            # 删除name_orig，恢复orig
            if not isinstance(method, PruningContainer):
                orig = getattr(module, name + "_orig")
                module.register_parameter(name, orig)
                del module._parameters[name + "_orig"]
            raise e

        return method

可以看到，BasePruningMethod基类中，抽象方法compute_mask()、__init__ 需要子类进行实现，apply()方法可以调用基类的方法即可。

L1Unstructured为例

class L1Unstructured(BasePruningMethod):
    r"""非结构化，最小L1norm（绝对值）的值zero out.
    amount，要裁剪参数的比率，如果是整数，则是裁剪的参数总个数
    """

    PRUNING_TYPE = "unstructured" ##必须指明结构化还是非结构化

    def __init__(self, amount):
        # Check range of validity of pruning amount
        _validate_pruning_amount_init(amount)
        self.amount = amount

    ## 重写父类方法compute_mask
    def compute_mask(self, t, default_mask):
        ..."""  """
        # 计算要裁剪的参数个数
        nparams_toprune = _compute_nparams_toprune(self.amount, tensor_size)
        ...

        # container接口里的default_mask本地拷贝一下
        mask = default_mask.clone(memory_format=torch.contiguous_format)

        if nparams_toprune != 0:  # k=0 not supported by torch.kthvalue
            # largest=True --> top k; largest=False --> bottom k
            # 取出abs最小的那些权重序号，将对应的mask位置置为0
            topk = torch.topk(torch.abs(t).view(-1), k=nparams_toprune, largest=False)
            # topk will have .indices and .values
            mask.view(-1)[topk.indices] = 0

        return mask

    @classmethod
    def apply(cls, module, name, amount, importance_scores=None):
        r"""调用父类方法，增加forward pre-hook 方便来做reparametrization ，生成 original tensor(xxx_orig)
        和pruning mask(xxx_mask).
        输入：module，module的参数名，要剪的比率.
        """
        # 调用父类的BasePruningMethod的apply方法
        return super(L1Unstructured, cls).apply(
            module, name, amount=amount, importance_scores=importance_scores
        )

PruningContainer

PruningContainer这个类同样也是继承自BasePruningMethod类，它的作用主要是对剪枝策略进行管理。
既然是继承自BasePruningMethod类，必然要实现__init__、compute_mask，此外，该类中单独实现了add_pruning_method用于储存裁剪策略，其中slc 存放的是非结构化剪枝的元素位置/结构化剪枝中的保留通道信息

class PruningContainer(BasePruningMethod):
    """迭代迭代pruning的方法类.
    记录BasePruningMethod的序列，然后pruning时按照顺序来apply这些BasePruningMethod
    输入为：BasePruningMethod继承子类对象
    """

    def __init__(self, *args):
        self._pruning_methods: Tuple["BasePruningMethod", ...] = tuple()
        ...
                self.add_pruning_method(method)

    def add_pruning_method(self, method):
        r"""
        输入为：BasePruningMethod继承子类对象
        """
        ...
        # if all checks passed, add to _pruning_methods tuple
        self._pruning_methods += (method,)

    ...

    # 迭代多次pruning
    def compute_mask(self, t, default_mask):
        r""" new mask 根据 ``PRUNING_TYPE`` ，因为mask的地方在后续就不参与统计计算了嘛:
        * 'unstructured', 非结构化，mask基于nonmasked位置来叠加生成;
        * 'structured', 结构化，mask 根据没有zero-out的channel来叠加;
        * 'global', 非结构化，全局的，所以是根据整体的所有元素来统计.
        输入：t，待裁剪的parameter，和default_mask维度同
        default_mask，迭代剪枝当前的mask值
        返回：default_mask和对当前剪枝method获取的new_mask合成
        """

        def _combine_masks(method, t, mask):
            r"""
            Args:
                method BasePruningMethod的实例
                t (torch.Tensor): 需要剪的tensor.
                mask (torch.Tensor): 历史mask
            Returns:
                new_mask (torch.Tensor): 合并之后的新mask.
            """
            new_mask = mask  # start off from existing mask
            new_mask = new_mask.to(dtype=t.dtype)

            # slc 存放的是非结构化剪枝的元素位置/结构化剪枝中的保留通道信息
            # compute a slice of t onto which the new pruning method will operate
            if method.PRUNING_TYPE == "unstructured":
                # mask tensor上为1的地方
                # 非结构化剪枝
                slc = mask == 1

            # for struct pruning, exclude channels that have already been
            # entirely pruned
            elif method.PRUNING_TYPE == "structured":
                if not hasattr(method, "dim"):
                    raise AttributeError(
                        "Pruning methods of PRUNING_TYPE "
                        '"structured" need to have the attribute `dim` defined.'
                    )

                # find the channels to keep by removing the ones that have been
                # zeroed out already (i.e. where sum(entries) == 0)
                n_dims = t.dim()  # "is this a 2D tensor? 3D? ..."
                dim = method.dim
                # convert negative indexing
                if dim < 0:
                    dim = n_dims + dim
                # if dim is still negative after subtracting it from n_dims
                if dim < 0:
                    raise IndexError(
                        "Index is out of bounds for tensor with dimensions {}".format(
                            n_dims
                        )
                    )
                # find channels along dim = dim that aren't already tots 0ed out
                # 统计mask里是否全0，keep_channel为method设置的dim里没有全部zero-out的通道
                keep_channel = mask.sum(dim=[d for d in range(n_dims) if d != dim]) != 0
                # create slice to identify what to prune
                slc = [slice(None)] * n_dims
                slc[dim] = keep_channel

            elif method.PRUNING_TYPE == "global":
                # 非结构化剪枝
                n_dims = len(t.shape)  # "is this a 2D tensor? 3D? ..."
                slc = [slice(None)] * n_dims

            else:
                raise ValueError(
                    "Unrecognized PRUNING_TYPE {}".format(method.PRUNING_TYPE)
                )

            # compute the new mask on the unpruned slice of the tensor t
            # 具体调用每种方法的compute_mask与default_mask一起生成新的mask
            partial_mask = method.compute_mask(t[slc], default_mask=mask[slc])
            new_mask[slc] = partial_mask.to(dtype=new_mask.dtype)

            return new_mask

        # 从序列头上里取出method，_combine_masks调用该method的compute_mask
        method = self._pruning_methods[-1]
        mask = _combine_masks(method, t, default_mask)
        return mask

在compute_mask函数的实现中，非结构化剪枝时，将mask tensor上为1 的地方保存在slc，同理，对于结构化剪枝，会通过统计mask里是否全为0,并将method设置的dim里没有全部zero-out的通道保存在keep_channel,赋值给slc,然后具体调用每种方法的compute_mask与default_mask一起生成新的mask并返回。

Pruning Method

• prune(self, t, default_mask=None, importance_scores=None)
  同样，该函数也为基类BasePruningMethod的类方法，通过调用调用compute_mask，返回pruned之后的tensor

def prune(self, t, default_mask=None, importance_scores=None):
        r"""调用compute_mask，返回pruned之后的tensor        
        """
        ...
        return t * self.compute_mask(importance_scores, default_mask=default_mask)

• emove(self, module)
  这个类方法的作用就是将参数的缓存和mask都去掉，永久化剪枝，不可逆

    def remove(self, module):
        r"""将参数的缓存和mask都去掉，永久化剪枝. parameter
         ``name+'_orig'`` 从 parameter list顺出.  ``name+'_mask'`` 从 buffers删除
         不可逆
        """
        # 是否已经设置过pruning
        assert (
            self._tensor_name is not None
        ), "Module {} has to be pruned\
            before pruning can be removed".format(
            module
        )  # this gets set in apply()

        # to update module[name] to latest trained weights
        weight = self.apply_mask(module)  # masked weights

        # 删除原来的weight，替换为apply_mask
        if hasattr(module, self._tensor_name):
            delattr(module, self._tensor_name)
        orig = module._parameters[self._tensor_name + "_orig"]
        orig.data = weight.data
        # 删除name_orig和name_mask
        del module._parameters[self._tensor_name + "_orig"]
        del module._buffers[self._tensor_name + "_mask"]
        setattr(module, self._tensor_name, orig)