Can't convert to fp16 for detectnet_v2_resnet18 trained model using TLT on desktop gtx 1080

Inside docker,please use tlt-export to generate etlt model. Actually all the etlt model is fp32 mode.See Difference in data type specified during tlt export and tlt convert - #5 by Morganh

In nano, please use tlt-converter to generate fp16 trt engine.
Nano supports fp16.