Do not use upper cased variables

[pavanky]

Considering that thrust is an upstream project for many others, using cased names, especially when they are generic, should be avoided.

For example the following link uses the variable SMEM which is an example of something that is likely to be used as a macro.

https://github.com/thrust/thrust/blob/master/thrust/system/cuda/detail/reduce_by_key.inl