Fix return type

Author: scotts

src/torchcodec/decoders/_core/video_decoder_ops.py

@@ -259,8 +259,8 @@ def get_frames_by_pts_in_range_abstract(
 @register_fake("torchcodec_ns::_get_key_frame_indices")
 def get_key_frame_indices_abstract(
     decoder: torch.Tensor, *, stream_index: int
-) -> List[int]:
-    return []
+) -> torch.Tensor:
+    return torch.empty([], dtype=torch.int)
 
 
 @register_fake("torchcodec_ns::get_json_metadata")