Feature Request: ALS GPU Acceleration using CUBLAS or OpenCL

[Skylion007]

#A welcome addition to this repository would be to offload the ALS operations to the GPU. There exist several libraries which allow Julia to offload many of these operations to the GPU. Furthermore, Nivida has recently released an ALS implementation in CUDA. Adding GPU support would significantly speed up the training process.

[ViralBShah]

Submit a PR?