Merging forward and backward kernels into one

[novakovicdj]

Replaced two kernels with one because they are similar. Merged two dropout functions that are calling kernels into one with parameter is_backward to determine which kernel should be executed.

[junliume]

@novakovicdj could you help with the above mentioned issue? Thanks!

[novakovicdj]

@junliume hi, can you review this one again? Thanks