Create date_modified() (ultralytics#2616)

Updated device selection string with fallback for non-git directories.
```python
def select_device(device='', batch_size=None):
    # device = 'cpu' or '0' or '0,1,2,3'
    s = f'YOLOv5 🚀 {git_describe() or date_modified()} torch {torch.__version__} '  # string
...
```

utils/torch_utils.py

@@ -1,4 +1,6 @@
-# PyTorch utils
+# YOLOv5 PyTorch utils
+
+import datetime
 import logging
 import math
 import os
@@ -43,9 +45,15 @@ def init_torch_seeds(seed=0):
         cudnn.benchmark, cudnn.deterministic = True, False
 
 
-def git_describe():
+def date_modified(path=__file__):
+    # return human-readable file modification date, i.e. '2021-3-26'
+    t = datetime.datetime.fromtimestamp(Path(path).stat().st_mtime)
+    return f'{t.year}-{t.month}-{t.day}'
+
+
+def git_describe(path=Path(__file__).parent):  # path must be a directory
     # return human-readable git description, i.e. v5.0-5-g3e25f1e https://git-scm.com/docs/git-describe
-    s = f'git -C {Path(__file__).resolve().parent} describe --tags --long --always'
+    s = f'git -C {path} describe --tags --long --always'
     try:
         return subprocess.check_output(s, shell=True).decode()[:-1]
     except subprocess.CalledProcessError as e:
@@ -54,7 +62,7 @@ def git_describe():
 
 def select_device(device='', batch_size=None):
     # device = 'cpu' or '0' or '0,1,2,3'
-    s = f'YOLOv5 🚀 {git_describe()} torch {torch.__version__} '  # string
+    s = f'YOLOv5 🚀 {git_describe() or date_modified()} torch {torch.__version__} '  # string
     cpu = device.lower() == 'cpu'
     if cpu:
         os.environ['CUDA_VISIBLE_DEVICES'] = '-1'  # force torch.cuda.is_available() = False