Enable direct --weights URL definition (ultralytics#3373)

* Enable direct `--weights URL` definition

@kalenmike this PR will enable direct --weights URL definition. Example use case:
```
python train.py --weights https://storage.googleapis.com/bucket/dir/model.pt
```

* cleanup

* bug fixes

* weights = attempt_download(weights)

* Update experimental.py

* Update hubconf.py

* return bug fix

* comment mirror

* min_bytes

(cherry picked from commit ba6f3f9)

utils/google_utils.py

@@ -4,7 +4,6 @@
 import platform
 import subprocess
 import time
-import urllib
 from pathlib import Path
 
 import requests
@@ -20,32 +19,30 @@ def gsutil_getsize(url=''):
 def safe_download(file, url, url2=None, min_bytes=1E0, error_msg=''):
     # Attempts to download file from url or url2, checks and removes incomplete downloads < min_bytes
     file = Path(file)
-    assert_msg = f"Downloaded file '{file}' does not exist or size is < min_bytes={min_bytes}"
-    try:  # url1
+    try:  # GitHub
         print(f'Downloading {url} to {file}...')
         torch.hub.download_url_to_file(url, str(file))
-        assert file.exists() and file.stat().st_size > min_bytes, assert_msg  # check
-    except Exception as e:  # url2
+        assert file.exists() and file.stat().st_size > min_bytes  # check
+    except Exception as e:  # GCP
         file.unlink(missing_ok=True)  # remove partial downloads
-        print(f'ERROR: {e}\nRe-attempting {url2 or url} to {file}...')
+        print(f'Download error: {e}\nRe-attempting {url2 or url} to {file}...')
         os.system(f"curl -L '{url2 or url}' -o '{file}' --retry 3 -C -")  # curl download, retry and resume on fail
     finally:
         if not file.exists() or file.stat().st_size < min_bytes:  # check
             file.unlink(missing_ok=True)  # remove partial downloads
-            print(f"ERROR: {assert_msg}\n{error_msg}")
+            print(f'ERROR: Download failure: {error_msg or url}')
         print('')
 
 
-def attempt_download(file, repo='ultralytics/yolov5'):  # from utils.google_utils import *; attempt_download()
+def attempt_download(file, repo='ultralytics/yolov5'):
     # Attempt file download if does not exist
     file = Path(str(file).strip().replace("'", ''))
 
     if not file.exists():
         # URL specified
-        name = Path(urllib.parse.unquote(str(file))).name  # decode '%2F' to '/' etc.
+        name = file.name
         if str(file).startswith(('http:/', 'https:/')):  # download
             url = str(file).replace(':/', '://')  # Pathlib turns :// -> :/
-            name = name.split('?')[0]  # parse authentication https://url.com/file.txt?auth...
             safe_download(file=name, url=url, min_bytes=1E5)
             return name