check_requirements() exclude pycocotools, thop (ultralytics#2571)

Exclude non-critical packages from dependency checks in detect.py. pycocotools and thop in particular are not required for inference.

Issue first raised in ultralytics#1944 and also raised in ultralytics#2556

(cherry picked from commit 0d891c6)

detect.py

@@ -164,7 +164,7 @@ def detect(save_img=False):
     parser.add_argument('--exist-ok', action='store_true', help='existing project/name ok, do not increment')
     opt = parser.parse_args()
     print(opt)
-    check_requirements()
+    check_requirements(exclude=('pycocotools', 'thop'))
 
     with torch.no_grad():
         if opt.update:  # update all models (to fix SourceChangeWarning)