[dialogpt] conversion script

Reference: https://github.com/huggingface/transformers/pull/1778#issuecomment-567675530

cc @patrickvonplaten and @dreasysnail

src/transformers/convert_dialogpt_original_pytorch_checkpoint_to_pytorch.py

+import argparse
+import os
+
+import torch
+
+from transformers.file_utils import WEIGHTS_NAME
+
+
+DIALOGPT_MODELS = ["small", "medium", "large"]
+
+OLD_KEY = "lm_head.decoder.weight"
+NEW_KEY = "lm_head.weight"
+
+
+def convert_dialogpt_checkpoint(checkpoint_path: str, pytorch_dump_folder_path: str):
+    d = torch.load(checkpoint_path)
+    d[NEW_KEY] = d.pop(OLD_KEY)
+    os.makedirs(pytorch_dump_folder_path, exist_ok=True)
+    torch.save(d, os.path.join(pytorch_dump_folder_path, WEIGHTS_NAME))
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--dialogpt_path", default=".", type=str)
+    args = parser.parse_args()
+    for MODEL in DIALOGPT_MODELS:
+        checkpoint_path = os.path.join(args.dialogpt_path, f"{MODEL}_ft.pkl")
+        pytorch_dump_folder_path = f"./DialoGPT-{MODEL}"
+        convert_dialogpt_checkpoint(
+            checkpoint_path, pytorch_dump_folder_path,
+        )