Speed up GPU computation on EI & gradEI

[jialeiwang]

The original GPU implementation is slow, which is due to both malloc and operating on global memory for normal random numbers. To make the improvement, we should not use malloc for each thread, instead, we create shared memory within each block to hold random numbers, and let threads within that block read/write random numbers stored in shared memory.

The other potential improvement is instead of copying (no_of_blocks * no_of_threads) EI or grad_EI from GPU to CPU and averaging them, we can average them on GPU and then simply copy a single piece of EI or grad_EI back to CPU. This improvement has been proven to be negligible for reasonable large q and p(q=4, p=4). We might need to look back again to see if this improvement is worth to implement for large q,p.