Added `YOLOv5_AUTOINSTALL` environment variable (#7505)

* Added a way to skip dependency auto-installation.

Setting the environment variable `YOLOv5_AUTOINSTALL=False` will
skip installing any missing dependencies as if the user had passed
`install=False` to `check_requirements`.

* Cleanup

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/general.py

@@ -40,6 +40,7 @@ FILE = Path(__file__).resolve()
 ROOT = FILE.parents[1]  # YOLOv5 root directory
 DATASETS_DIR = ROOT.parent / 'datasets'  # YOLOv5 datasets directory
 NUM_THREADS = min(8, max(1, os.cpu_count() - 1))  # number of YOLOv5 multiprocessing threads
+AUTOINSTALL = str(os.getenv('YOLOv5_AUTOINSTALL', True)).lower() == 'true'  # global auto-install mode
 VERBOSE = str(os.getenv('YOLOv5_VERBOSE', True)).lower() == 'true'  # global verbose mode
 FONT = 'Arial.ttf'  # https://ultralytics.com/assets/Arial.ttf
 
@@ -338,7 +339,7 @@ def check_requirements(requirements=ROOT / 'requirements.txt', exclude=(), insta
             pkg.require(r)
         except Exception:  # DistributionNotFound or VersionConflict if requirements not met
             s = f"{prefix} {r} not found and is required by YOLOv5"
-            if install:
+            if install and AUTOINSTALL:  # check environment variable
                 LOGGER.info(f"{s}, attempting auto-update...")
                 try:
                     assert check_online(), f"'pip install {r}' skipped (offline)"