Support int8 transposed convolutions with per-channel weight quantization

TFLite uses int8 per-channel weight quantization for transposed convolutions.
While XNNPACK includes a fast transposed convolution operation it only supports per-tensor weight quantization (i.e. a single quantisation scale for the weight tensor), which means transposed convolutions in a TFLite QAT int8 model are currently not supported by XNNPACK and won't be accelerated.

It would be excellent if XNNPACK would add support for per-channel quantized weights to the transposed convolution op in the future to match the behaviour of the normal convolution.