Save Model: support TorchScript

Various software requires models in TorchScript format. As one example,
this can be used to convert PyTorch models to ncnn models via PNNX,
without using ONNX as an intermediary.

backend/src/packages/chaiNNer_pytorch/pytorch/io/save_model.py

@@ -21,6 +21,7 @@
 class WeightFormat(Enum):
     PTH = "pth"
     ST = "safetensors"
+    PT = "pt"
 
 
 @io_group.register(
@@ -42,6 +43,7 @@ class WeightFormat(Enum):
             option_labels={
                 WeightFormat.PTH: "PyTorch (.pth)",
                 WeightFormat.ST: "SafeTensors (.safetensors)",
+                WeightFormat.PT: "TorchScript (.pt)",
             },
         ),
     ],
@@ -58,5 +60,13 @@ def save_model_node(
         torch.save(model.model.state_dict(), full_path)
     elif weight_format == WeightFormat.ST:
         save_file(model.model.state_dict(), full_path)
+    elif weight_format == WeightFormat.PT:
+        size = 3
+        size += model.size_requirements.get_padding(size, size)[0]
+        dummy_input = torch.rand(1, model.input_channels, size, size)
+        dummy_input = dummy_input.to(model.device)
+
+        trace = torch.jit.trace(model.model, example_inputs=dummy_input)
+        trace.save(full_path)
     else:
         raise ValueError(f"Unknown weight format: {weight_format}")