Suppress export.run() TracerWarnings (ultralytics#6499)

Suppresses warnings when calling export.run() directly, not just CLI python export.py.

Also adds Requirements examples for CPU and GPU backends

export.py

@@ -16,6 +16,10 @@
 TensorFlow Edge TPU         | `edgetpu`                     | yolov5s_edgetpu.tflite
 TensorFlow.js               | `tfjs`                        | yolov5s_web_model/
 
+Requirements:
+    $ pip install -r requirements.txt coremltools onnx onnx-simplifier onnxruntime openvino-dev tensorflow-cpu  # CPU
+    $ pip install -r requirements.txt coremltools onnx onnx-simplifier onnxruntime-gpu openvino-dev tensorflow  # GPU
+
 Usage:
     $ python path/to/export.py --weights yolov5s.pt --include torchscript onnx openvino engine coreml tflite ...
 
@@ -437,6 +441,7 @@ def run(data=ROOT / 'data/coco128.yaml',  # 'dataset.yaml path'
 
     # Exports
     f = [''] * 10  # exported filenames
+    warnings.filterwarnings(action='ignore', category=torch.jit.TracerWarning)  # suppress TracerWarning
     if 'torchscript' in include:
         f[0] = export_torchscript(model, im, file, optimize)
     if 'engine' in include:  # TensorRT required before ONNX
@@ -509,10 +514,8 @@ def parse_opt():
 
 
 def main(opt):
-    with warnings.catch_warnings():
-        warnings.filterwarnings(action='ignore', category=torch.jit.TracerWarning)  # suppress TracerWarning
-        for opt.weights in (opt.weights if isinstance(opt.weights, list) else [opt.weights]):
-            run(**vars(opt))
+    for opt.weights in (opt.weights if isinstance(opt.weights, list) else [opt.weights]):
+        run(**vars(opt))
 
 
 if __name__ == "__main__":