See: http://pytorch.org/docs/0.4.0/onnx.html
The other frameworks are using (in my opinion) slightly misleading language as to how "ready" it is. onnx to Caffe2 is facebook's use case so I could see that being supported fine. It's hard to go much beyond that though.
Support for onnx will be bottlenecked by what pytorch can export right now. The file format just hit 1.0, but it will take some time for the ecosystem around it (including the export) to mature.
Disclaimer: I am a framework vendor who has spent the last few months messing with it for end users writing model import
for tensorflow's file format as well.
ONNX to Caffe2*. Though it's possible to go from Caffe2 to ONNX to some custom hardware accelerator (TensorRT?)
And it will be possible but again support isn't actually implemented by anyone yet.
They just "signed on".
Tensorflow is the only supported format right now for tensorrt.