DepthwiseConv2dNative

Computes a 2-D depthwise convolution given 4-D `input` and `filter` tensors.

Given an input tensor of shape `[batch, in_height, in_width, in_channels]` and a filter / kernel tensor of shape `[filter_height, filter_width, in_channels, channel_multiplier]`, containing `in_channels` convolutional filters of depth 1, `depthwise_conv2d` applies a different filter to each input channel (expanding from 1 channel to `channel_multiplier` channels for each), then concatenates the results together. Thus, the output has `in_channels * channel_multiplier` channels.

for k in 0..in_channels-1
   for q in 0..channel_multiplier-1
     output[b, i, j, k * channel_multiplier + q] =
       sum_{di, dj input[b, strides[1] * i + di, strides[2] * j + dj, k] *
                         filter[di, dj, k, q]
 }