Fixing repo_id regex validation error on windows platforms (#4358)

* Fixing repo_id regex validation error on windows platforms

* Validating correct URL with prefix is provided

If we are loading a URL then we don't need to use os.path.join and array slicing to split out a repo_id and file path from an absolute filepath. 

Checking if the URL prefix is valid first before doing any URL splitting otherwise we raise a ValueError since neither a valid filepath or URL was provided.

* Style fixes

---------
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/loaders.py

@@ -1877,16 +1877,24 @@ class FromSingleFileMixin:
             raise ValueError(f"Unhandled pipeline class: {pipeline_name}")
 
         # remove huggingface url
-        for prefix in ["https://huggingface.co/", "huggingface.co/", "hf.co/", "https://hf.co/"]:
+        has_valid_url_prefix = False
+        valid_url_prefixes = ["https://huggingface.co/", "huggingface.co/", "hf.co/", "https://hf.co/"]
+        for prefix in valid_url_prefixes:
             if pretrained_model_link_or_path.startswith(prefix):
                 pretrained_model_link_or_path = pretrained_model_link_or_path[len(prefix) :]
+                has_valid_url_prefix = True
 
         # Code based on diffusers.pipelines.pipeline_utils.DiffusionPipeline.from_pretrained
         ckpt_path = Path(pretrained_model_link_or_path)
         if not ckpt_path.is_file():
+            if not has_valid_url_prefix:
+                raise ValueError(
+                    f"The provided path is either not a file or a valid huggingface URL was not provided. Valid URLs begin with {', '.join(valid_url_prefixes)}"
+                )
+
             # get repo_id and (potentially nested) file path of ckpt in repo
-            repo_id = os.path.join(*ckpt_path.parts[:2])
-            file_path = os.path.join(*ckpt_path.parts[2:])
+            repo_id = "/".join(ckpt_path.parts[:2])
+            file_path = "/".join(ckpt_path.parts[2:])
 
             if file_path.startswith("blob/"):
                 file_path = file_path[len("blob/") :]