Parsing Relay subgraph with composite function under BYOC

By pattern annotation: qnn.op.conv2d+nn.bias_add+qnn.op.requantize, it corresponds to a complete INT8 CONV with bias layer in Caffe. The code is too specific to the target hardware and I don’t think it’s more valuable than a generalized float32 conversion one. However, for fixed point accelerator, it should be always a requantize op followed at the end.