YOLOv5 PyTorch Hub models >> check_requirements() (ultralytics#2577)

* Update hubconf.py with check_requirements()

Dependency checks have been missing from YOLOv5 PyTorch Hub model loading, causing errors in some cases when users are attempting to import hub models in unsupported environments. This should examine the YOLOv5 requirements.txt file and pip install any missing or version-conflict packages encountered. 

This is highly experimental (!), please let us know if this creates problems in your custom workflows.

* Update hubconf.py

(cherry picked from commit 2bcc89d)

hubconf.py

@@ -1,20 +1,21 @@
-"""File for accessing YOLOv5 via PyTorch Hub https://pytorch.org/hub/
+"""File for accessing YOLOv5 models via PyTorch Hub https://pytorch.org/hub/ultralytics_yolov5/
 
 Usage:
     import torch
-    model = torch.hub.load('ultralytics/yolov5', 'yolov5s', pretrained=True, channels=3, classes=80)
+    model = torch.hub.load('ultralytics/yolov5', 'yolov5s')
 """
 
 from pathlib import Path
 
 import torch
 
 from models.yolo import Model
-from utils.general import set_logging
+from utils.general import check_requirements, set_logging
 from utils.google_utils import attempt_download
 from utils.torch_utils import select_device
 
 dependencies = ['torch', 'yaml']
+check_requirements(exclude=('pycocotools', 'thop'))
 set_logging()