Improve DyNet usage

[LoicGrobol]

We can take better advantage of what DyNet offers

• Activating autobatching (easy to do and seems to improve speed in at least my anecdotal benchmark :-)
• Remove noop dy.parameter calls

[mdelhoneux]

Just saw this, regarding autobatching, I tried to get it to work a while ago (2 years?) and accuracy took a hit :/ talked to Yoav Goldberg about it who did not seem surprised

[LoicGrobol]

Ah, too bad. Did he have an idea why?

[mdelhoneux]

Well batching is good for efficiency not accuracy so with a larger batch size, you need to train for more epochs if you want to reach the same result as with no batch at all. I didn't experiment so much so it might still be worth doing, especially if you want to train models on multiple treebanks, but you probably will need to train for more epochs and might not gain so much in training speed in the end