* Added op::AvgPool cudnn impl. which works for 2-3 spatial dimesions and no/symmetric padding. Enabled tests.

* Added cuda-c implementation of average pool which handles 1-3 spatial
dimensions as well as asymmetric padding. This commit also introduces
several helper functions for performing fast integer division and
fast constant memory access.

* Formatting. Removed bool that was used for testing to force the cuda impl. over cudnn.

* Added CUDNN AvgPoolBackprop implementation.

* Removed inline enum in preference of a helper struct. Removed instances of multiple declarations on a single line. Updated comments.

* Removed _prefix to helper functions in anonymous namespace.