Update vsmlrt.py:

• Introduce a new release artifact ext-models.v12.2.7z, which comes from External Models, and it's not bundled into full binary release packages (i.e. the cpu, cuda and vk packages). Please refer to their release notes for details on how to use those models.

• Export a new API vsmlrt.inference for inference of custom models.

  import vsmlrt
  output = vsmlrt.inference(clips, "path/to/onnx", backend=vsmlrt.Backend.TRT(fp16=True))

  If you encounter issues like Cannot find input tensor with name "input" in the network inputs! Please make sure the input tensor names are correct., you could use vsmlrt.inference(..., input_name=None) or export the model with its input name set to "input".

• Fix trt inference of cugan-pro (3x) models. (#15)