ONNX Simplifier (#2815)

* ONNX Simplifier

Add ONNX Simplifier to ONNX export pipeline in export.py. Will auto-install onnx-simplifier if onnx is installed but onnx-simplifier is not.

* Update general.py

utils/general.py

@@ -111,7 +111,7 @@ def check_requirements(requirements='requirements.txt', exclude=()):
         except Exception as e:  # DistributionNotFound or VersionConflict if requirements not met
             n += 1
             print(f"{prefix} {e.req} not found and is required by YOLOv5, attempting auto-update...")
-            print(subprocess.check_output(f"pip install '{e.req}'", shell=True).decode())
+            print(subprocess.check_output(f"pip install {e.req}", shell=True).decode())
 
     if n:  # if packages updated
         source = file.resolve() if 'file' in locals() else requirements