Dataset parameters setting

Thank you for your contribution~
In the code `dataset/Endovis2017.py`, `MEAN` and `STD` are set as fixed number. 
```
MEAN = [0.40789654, 0.44719302, 0.47026115]
STD = [0.28863828, 0.27408164, 0.27809835]
```
How do you determine the parameters in practice?
By the way, can you update the `train.py`? The current one does not seem to use `DMNet`.