Update default ext from npz to pkl in body_model_defaults.py

[syKevinPeng]

The current transfer model script requires .npz model file instead of pkl. Updating this to reflect current file ext as described in the readme.

[syKevinPeng]

Issue a):
File: transfer_model.py
If statement starting from line 345 was never triggered, which result of "None" output for translation optimization. There is not "translation" key in var_dict, but transl (which is initialized in line 216).

Issue b):
Current default file extention for simple model is .pkl. However, the default config for transfering SMPL to SMPLH is still using .npz.