* Refactored GPU backend state into BackendContext and moved it to the highest level GPU_Backend.
Some bugs have appeared in so doing. Needs investigation.

* extra *block_size

* change grid_size to threads

* Bug fix in softmax cache parameters.

* Additional bug fix for maxpool1d cache parameters.

* Bug fix in softmax cache parameters.

* Additional bug fix for maxpool1d cache parameters.

* Remove temporary print statements.

* Use nthreads in primitive hash.

* Switched from using stack references for cudnn and cublas handles to heap pointers held only the c-struct GPURuntimeContext but managed by the GPU_Backend.

* Refactored the use of GPURuntimeContext* ctx throughout the emitters.

* Use std::prev instead of operator-- for memory iteratory capture

* bug fix from abaf1d7