the typical usage
```
ncnnoptimize mobilenet.param mobilenet.bin mobilenet-opt.param mobilenet-opt.bin 65536 
```

operator fusion
* batchnorm - scale
* convolution - batchnorm
* convolutiondepthwise - batchnorm
* deconvolution - batchnorm
* deconvolutiondepthwise - batchnorm
* innerproduct - batchnorm
* convolution - relu
* convolutiondepthwise - relu
* deconvolution - relu
* deconvolutiondepthwise - relu
* innerproduct - relu

eliminate noop operator
* innerproduct - dropout
* flatten after global pooling

prefer better operator
* replace convolution with innerproduct after global pooling