expose backend selection and cudnn settings to the end user

[soumith]

This is needed for the advanced user, where one might need to choose a specific backend, or a particular algorithm / mode in cudnn.

cc @csarofeen @ptrblck @xwang233

[supakjk]

I wonder if this issue will be dealt or not. Since I'd like to use different backends fo different modules in a single model, (e.g., using cudnn for LSTM but not using for ConvNet) it would be great if this issue is resolved.

[ezyang]

This is still a valid issue! Primary blocker for it is coming up with a good API for specifying what algorithm you want.