make densepose inference model traceable

Summary: make the densepose_inference function tracable.

Reviewed By: bilalsal

Differential Revision: D40399377

fbshipit-source-id: 758792ab7dcc58a9b1d39cf482a2067e9ab6bbbf

Author: Wei Wang

projects/DensePose/densepose/modeling/inference.py

@@ -27,7 +27,8 @@ def densepose_inference(densepose_predictor_output: Any, detections: List[Instan
         if densepose_predictor_output is None:
             # don't add `pred_densepose` attribute
             continue
-        n_i = len(detection_i)
+        n_i = detection_i.__len__()
+
         PredictorOutput = type(densepose_predictor_output)
         output_i_dict = {}
         # we assume here that `densepose_predictor_output` is a dataclass object