set default workspaceIndexMode to image index for 3D pooling

[ce1adon]

Fix for issue #365

[daniellowell]

@ce1adon This is straight forward. Questions: Is this a breaking change for MIOpen? What was the previous behavior and what is the impact to PyTorch and Caffe2?

[ce1adon]

@daniellowell
When we added 3D pooling, we didn't have this feature workspaceIndexMode. At that time, 2D pooling adopted mask mode while 3D pooling adopted image mode. Later framework team required us to change 2D pooling to image mode for their unit tests, so we added workspaceIndexMode for user to select index mode and we set the default mode as mask mode for all. However, 3D pooling does not use mask mode. User need to use the new API miopenSetPoolingWorkSpaceIndexMode() to set the workspaceIndexMode to miopenPoolingWorkspaceIndexImage. If not, it will trigger the issue reported. For convenience, the PR set default workspaceIndexMode to image mode for 3D pooling. User can still use miopenSetPoolingWorkSpaceIndexMode() to change the index mode for both 2D and 3D pooling.