Question

为了我的教育，我正在尝试在卷积神经网络中实现N维卷积层。

我想实现反向传播功能。但是，我不确定这样做的最有效方法。

目前，我正在使用signal.fftconvolve来

在转发步骤中，对过滤器进行卷积，内核对所有过滤器进行转发；
在“反向传播”步骤中，对所有过滤器的导数（使用FlipAllAxes函数在所有维度上进行反转）与数组（https://jefkine.com/general/2016/09/05/backpropagation-in-convolutional-neural-networks/）进行卷积并求和。我得出的输出是每个图像与每个滤镜的每个导数卷积的总和。

我对如何对导数进行卷积感到特别困惑。使用下面的类向后传播会导致权重大小爆炸。

用输出和滤波器对导数进行卷积编程的正确方法是什么？

编辑：

根据这篇论文（Fast Training of Convolutional Networks through FFTs），该论文力求完全按照我的意愿去做：

上一层的导数由当前层的导数与权重的卷积给出：

dL / dy_f = dL / dx * w_f ^ T
权重的导数是导数与原始输入的卷积的分段总和：

dL / dy = dL / dx * x

据我所知，我已经在下面实现了这一点。但是，这似乎并没有达到预期的效果，因为我使用该层编写的网络在训练过程中表现出剧烈的波动。

    import numpy as np
    from scipy import signal

    class ConvNDLayer:
        def __init__(self,channels, kernel_size, dim):

            self.channels = channels
            self.kernel_size = kernel_size;
            self.dim = dim

            self.last_input = None

            self.filt_dims = np.ones(dim+1).astype(int)
            self.filt_dims[1:] =  self.filt_dims[1:]*kernel_size
            self.filt_dims[0]= self.filt_dims[0]*channels 
            self.filters = np.random.randn(*self.filt_dims)/(kernel_size)**dim


        def FlipAllAxes(self, array):

            sl = slice(None,None,-1)
            return array[tuple([sl]*array.ndim)] 

        def ViewAsWindows(self, array, window_shape, step=1):
             # -- basic checks on arguments
             if not isinstance(array, cp.ndarray):
                 raise TypeError("`array` must be a Cupy ndarray")
             ndim = array.ndim
             if isinstance(window_shape, numbers.Number):
                  window_shape = (window_shape,) * ndim
             if not (len(window_shape) == ndim):
                   raise ValueError("`window_shape` is incompatible with `arr_in.shape`")

             if isinstance(step, numbers.Number):
                  if step < 1:
                  raise ValueError("`step` must be >= 1")
                  step = (step,) * ndim
             if len(step) != ndim:
                   raise ValueError("`step` is incompatible with `arr_in.shape`")

              arr_shape = array.shape
              window_shape = np.asarray(window_shape, dtype=arr_shape.dtype))

              if ((arr_shape - window_shape) < 0).any():
                   raise ValueError("`window_shape` is too large")

              if ((window_shape - 1) < 0).any():
                    raise ValueError("`window_shape` is too small")

               # -- build rolling window view
                    slices = tuple(slice(None, None, st) for st in step)
                    window_strides = array.strides
                    indexing_strides = array[slices].strides
                    win_indices_shape = (((array.shape -window_shape)
                    // step) + 1)

                 new_shape = tuple(list(win_indices_shape) + list(window_shape))
                 strides = tuple(list(indexing_strides) + list(window_strides))

                  arr_out = as_strided(array, shape=new_shape, strides=strides)

                  return arr_out

        def UnrollAxis(self, array, axis):
             # This so it works with a single dimension or a sequence of them
             axis = cp.asnumpy(cp.atleast_1d(axis))
             axis2 = cp.asnumpy(range(len(axis)))

             # Put unrolled axes at the beginning
             array = cp.moveaxis(array, axis,axis2)
             # Unroll
             return array.reshape((-1,) + array.shape[len(axis):])

        def Forward(self, array):

             output_shape =cp.zeros(array.ndim + 1)    
             output_shape[1:] =  cp.asarray(array.shape)
             output_shape[0]= self.channels 
             output_shape = output_shape.astype(int)
             output = cp.zeros(cp.asnumpy(output_shape))

             self.last_input = array

             for i, kernel in enumerate(self.filters):
                    conv = self.Convolve(array, kernel)
                    output[i] = conv

             return output


        def Backprop(self, d_L_d_out, learn_rate):

            d_A= cp.zeros_like(self.last_input)
            d_W = cp.zeros_like(self.filters)


           for i, (kernel, d_L_d_out_f) in enumerate(zip(self.filters, d_L_d_out)):

                d_A += signal.fftconvolve(d_L_d_out_f, kernel.T, "same")
                conv = signal.fftconvolve(d_L_d_out_f, self.last_input, "same")
                conv = self.ViewAsWindows(conv, kernel.shape)
                axes = np.arange(kernel.ndim)
                conv = self.UnrollAxis(conv, axes)  
                d_W[i] = np.sum(conv, axis=0)


           output = d_A*learn_rate
           self.filters =  self.filters - d_W*learn_rate
           return output

Answer 1

通常，将梯度与learn_rate相乘是不够的。

为了获得更好的性能并减少严重的波动，可使用优化程序通过诸如除以过去的几个梯度（RMSprop）之类的方法来缩放梯度。

更新还取决于错误，如果您单独为每个样本传递错误，通常会产生噪声，因此最好对多个样本（迷你批次）求平均值。

ND卷积反向传播

1 个答案: