[OMPX] Change `thread_dim` to `block_dim` and the original `block_dim` to `grid_dim`

There is no `threadDim` in CUDA. Instead, it is `blockDim`. Then the current
`blockDim` is `gridDim` in CUDA.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D157051