Export ONNX in a separate process

On ZeroGPU hardware the already imported spaces package replaces torch's
CUDA with an emulation, and torch.onnx.export failed on an operation it
did not intercept. A fresh process never imports spaces.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

onnx_backend.py

@@ -5,6 +5,8 @@ Hub, so it is rebuilt on each start (about a minute on CPU) and kept in LAYA_ONN
 """
 import os
 import statistics
+import subprocess
+import sys
 import time
 
 ONNX_DIR = os.environ.get("LAYA_ONNX_DIR", os.path.join(os.path.expanduser("~"), ".cache", "laya-onnx"))
@@ -51,7 +53,9 @@ def load(model_id):
     path = onnx_path_for(model_id)
     if not os.path.exists(path):
         start = time.perf_counter()
-        export(model_id, path)
+        # A separate process: on ZeroGPU hardware the `spaces` package, already imported here,
+        # replaces torch's CUDA with an emulation that makes the export fail.
+        subprocess.run([sys.executable, os.path.abspath(__file__), model_id, path], check=True)
         print(f"ONNX export of {model_id} took {time.perf_counter() - start:.0f} s", flush=True)
     return ONNXAgent(model_id, onnx_path=path)
 
@@ -100,3 +104,7 @@ def compare(engines, cases, repeats=3):
         "cpu_count": os.cpu_count(),
         "requests_per_engine": len(times[next(iter(times))]),
     }
+
+
+if __name__ == "__main__":
+    export(sys.argv[1], sys.argv[2])