[JIT] script if tracing fix (#40468)
Summary:
Currently, torchvision annotates `batched_nms` with `torch.jit.script` so the function gets compiled when it is traced and ONNX will work. Unfortunately, this means we are eagerly compiling batched_nms, which fails if torchvision isn't built with `torchvision.ops.nms`. As a result, torchvision doesn't work on torch hub right now.
`_script_if_tracing` could solve our problem here, but right now it does not correctly interact with recursive compilation. This PR fixes that bug.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40468
Reviewed By: jamesr66a
Differential Revision: D22195771
Pulled By: eellison
fbshipit-source-id: 83022ca0bab6d389a48a478aec03052c9282d2b7
