Enable cpu usage

[tejaswivg]

These changes fix some issues I had when torch is used without cuda (cpu only).

[Splendide-Imaginarius]

I implemented #561 as an alternative approach (wasn't intentionally duplicating your effort; I wrote my PR last week before yours was submitted). I like your approach of defaulting to CPU mode if CUDA isn't available, but I think it would be desirable to allow the user to explicitly choose CPU even if CUDA is available (as my PR does). Feel free to borrow pieces of my PR; seems desirable to combine the respective strengths of both PR's.

[tejaswivg]

I implemented #561 as an alternative approach (wasn't intentionally duplicating your effort; I wrote my PR last week before yours was submitted). I like your approach of defaulting to CPU mode if CUDA isn't available, but I think it would be desirable to allow the user to explicitly choose CPU even if CUDA is available (as my PR does). Feel free to borrow pieces of my PR; seems desirable to combine the respective strengths of both PR's.

@xinntao any thoughts on this?