add yolov6 c++ and yolov6 pybind (#16)

* update .gitignore

* Added checking for cmake include dir

* fixed missing trt_backend option bug when init from trt

* remove un-need data layout and add pre-check for dtype

* changed RGB2BRG to BGR2RGB in ppcls model

* add model_zoo yolov6 c++/python demo

* fixed CMakeLists.txt typos

* update yolov6 cpp/README.md

fastdeploy/download.py

@@ -18,6 +18,7 @@ import shutil
 import requests
 import time
 import zipfile
+import tarfile
 import hashlib
 import tqdm
 import logging