Pointwise kernel choose grid size based on number of CU

[asroy]

#261 (comment)

[rocking5566]

Elementwise and maxpool backward kernel suffer from this issue.
As discussed with @qianfengz , this might need to modify StreamConfig

[qianfengz]

I will submit a PR to solve the issue. And specifically, the P.R will provide an interface like

`int  getAvailableComputeUnitCount(StreamConfig & stream_cfg); `

The interface will call hipExtStreamGetCUMask() and count the masks to get the number of available CUs.

Also in the P.R, a hipDevice_t device_id = 0; will be added to the definition of StreamConfig. This is needed in case of some situation, more information need be queried from the device underlying the current stream.

[taylding-amd]

Closing the issue as the fix has already been merged.