Create Knet.Layers21

[denizyuret]

• [x] move atype to Knet.jl
• [ ] move param to Layers21.
• [x] implement Uniform(r) and short form U as init fn. Define fanin, fanout. Write the formula instead of using cryptic names.
• [ ] have atype and init be standard options in all layers.
• [ ] 7.2.14. cudnnGetAttnDescriptor()
• [ ] 7.2.15. cudnnGetMultiHeadAttnBuffers()
• [ ] 7.2.16. cudnnGetMultiHeadAttnWeights()
• [ ] Use convbiasadd