common: custom hf endpoint support (#12769)

* common: custom hf endpoint support

Add support for custom huggingface endpoints via HF_ENDPOINT environment variable

You can now specify a custom huggingface endpoint using the HF_ENDPOINT environment variable when using the --hf-repo flag, which works similarly to huggingface-cli's endpoint configuration.

Example usage:
HF_ENDPOINT=https://hf-mirror.com/ ./bin/llama-cli --hf-repo Qwen/Qwen1.5-0.5B-Chat-GGUF --hf-file qwen1_5-0_5b-chat-q2_k.gguf -p "The meaning to life and the universe is"

The trailing slash in the URL is optional:
HF_ENDPOINT=https://hf-mirror.com ./bin/llama-cli --hf-repo Qwen/Qwen1.5-0.5B-Chat-GGUF --hf-file qwen1_5-0_5b-chat-q2_k.gguf -p "The meaning to life and the universe is"

* Update common/arg.cpp

readability Improvement

Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>

* Apply suggestions from code review

---------

Co-authored-by: ベアトリーチェ <148695646+MakiSonomura@users.noreply.github.com>
Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>

Author: eternaphia

common/arg.cpp

@@ -656,9 +656,13 @@ static void common_params_handle_model(
                 }
             }
 
-            // TODO: allow custom host
-            model.url = "https://huggingface.co/" + model.hf_repo + "/resolve/main/" + model.hf_file;
-
+            std::string hf_endpoint = "https://huggingface.co/";
+            const char * hf_endpoint_env = getenv("HF_ENDPOINT");
+            if (hf_endpoint_env) {
+                hf_endpoint = hf_endpoint_env;
+                if (hf_endpoint.back() != '/') hf_endpoint += '/';
+            }
+            model.url = hf_endpoint + model.hf_repo + "/resolve/main/" + model.hf_file;
             // make sure model path is present (for caching purposes)
             if (model.path.empty()) {
                 // this is to avoid different repo having same file name, or same file name in different subdirs