Use CPU implementation of function but GPU for all other operations

[chaitanyamalaviya]

Hi everyone,

Is there a way to use the CPU implementation of a function while still using GPU for the remaining dynet operations?

[When I use a function that does not have a GPU implementation and set the dynet-gpu flag, I get an error saying 'module' object has no attribute '<the_function_name>'. ]

[neubig]

This is a good idea, but is basically reliant on multi-device support: #92

I'll close this as it's essentially a duplicate but I agree that this is important functionality to have.