Update elf to support pytorch 0.4

[alcinos]

This PR updates the code base to be somewhat compatible with the new version of Pytorch.
Note that the new Pytorch version includes fixes in important things like the multinomial distribution (which is used in ELF). Hence it can lead to a different behaviour.

A summary of the changes:

• Use .item() where appropriate. I may have missed some places, since I only tested this PR on Atari
• Remove volatile and use torch.no_grad instead
• Some stricter reshaping to cope with finickier dimension checks

Unfortunately, merging this PR means that ELF no longer works on 0.3 and earlier.

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