[XLA:GPU] Simplify launch dimensions estimate for non-kernel fusions.
[XLA:GPU] Simplify launch dimensions estimate for non-kernel fusions.

That estimate should not be used in production code, because fusions that don't implement KernelFusionInterface don't generate CUDA kernels and there is nothing to fuse. We keep this estimate as a fallback mechanism. We don't have any additional information about the kernel, so we can simplify the heuristic and assume we launch one thread per output element and 128 threads per block.
