A position issue at stage of stiching motions 

## conditon
when xform_from_parent_joint of the root joint is not an eye matrix

## solution
I solved this by changing the code

from:
https://github.com/facebookresearch/fairmotion/blob/f7a2d3eaf9eb8c3d72f3d79a92b0ab5844d4c55d/fairmotion/core/motion.py#L246
to:
```python
T0 = self.skel.get_joint(key).xform_from_parent_joint
```

it works for me, please check.