fix: fix checkpoint filename for the PyTorch backend (#1585)

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## Summary by CodeRabbit

- **Bug Fixes**
- Improved handling of model checkpoints during training to ensure
compatibility with different file suffixes (`.pb` and `.pth`).

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <[email protected]>

dpgen/generator/run.py

@@ -811,7 +811,13 @@ def run_train_dp(iter_index, jdata, mdata):
         elif training_finetune_model is not None:
             init_flag = f" --finetune old/init{suffix}"
         command = f"{train_command} train {train_input_file}{extra_flags}"
-        command = f"{{ if [ ! -f model.ckpt.index ]; then {command}{init_flag}; else {command} --restart model.ckpt; fi }}"
+        if suffix == ".pb":
+            ckpt_suffix = ".index"
+        elif suffix == ".pth":
+            ckpt_suffix = ".pt"
+        else:
+            raise RuntimeError(f"Unknown suffix {suffix}")
+        command = f"{{ if [ ! -f model.ckpt{ckpt_suffix} ]; then {command}{init_flag}; else {command} --restart model.ckpt; fi }}"
         command = f"/bin/sh -c {shlex.quote(command)}"
         commands.append(command)
         command = f"{train_command} freeze"