Fix bugs

Signed-off-by: Jael Gu <mengjia.gu@zilliz.com>

README.md

@@ -15,7 +15,7 @@ This operator uses Facebook's [Detectron2](https://github.com/facebookresearch/d
 ```python
 import towhee
 
-towhee.glob('./towhee.jpg') \
+towhee.glob('./example.jpg') \
       .image_decode() \
       .object_detection.detectron2(model_name='retinanet_resnet50') \
       .show()

__init__.py

@@ -1,3 +1,5 @@
+from .detectron import Detectron2
+
 
 def detectron2(*args, **kwargs):
     return Detectron2(*args, **kwargs)

requirements.txt

@@ -1,3 +1,4 @@
 torch >= 1.8
 torchvision >= 0.9.0
-https://github.com/facebookresearch/detectron2/archive/refs/tags/v0.6.tar.gz
+detectron2
+# https://github.com/facebookresearch/detectron2/archive/refs/tags/v0.6.tar.gz