Get TopK op working on GPU again. Extend using cub's radix sort.

1. Undo rollback of Andreas Kirsch's initial implementation.
2. Use cub segmented radix sort if Andreas' heap-based impl
   for large k and small num_cols (thresholds of k=100, n=1000
   determined empirically).
3. Use cub segmented radix sort if k == num_cols (this case is always faster).
4. Added benchmarks.

Benchmarks show that the GPU implementation is up to 3x slower for small k but
can be 10x faster for large num_cols and k.

Benchmarks:

Benchmark: m_128_n_10_k_5_use_gpu_False          wall_time: 0.000166 s   Throughput: 0.0077 GB/s
Benchmark: m_128_n_10_k_5_use_gpu_True   wall_time: 0.000796 s   Throughput: 0.00161 GB/s
Benchmark: m_128_n_10_k_9_use_gpu_False          wall_time: 0.00017 s    Throughput: 0.00751 GB/s
Benchmark: m_128_n_10_k_9_use_gpu_True   wall_time: 0.000796 s   Throughput: 0.00161 GB/s
Benchmark: m_128_n_10_k_10_use_gpu_False         wall_time: 0.00017 s    Throughput: 0.00753 GB/s
Benchmark: m_128_n_10_k_10_use_gpu_True          wall_time: 0.000775 s   Throughput: 0.00165 GB/s
Benchmark: m_128_n_100_k_1_use_gpu_False         wall_time: 0.000155 s   Throughput: 0.0826 GB/s
Benchmark: m_128_n_100_k_1_use_gpu_True          wall_time: 0.000796 s   Throughput: 0.0161 GB/s
Benchmark: m_128_n_100_k_50_use_gpu_False        wall_time: 0.000247 s   Throughput: 0.0519 GB/s
Benchmark: m_128_n_100_k_50_use_gpu_True         wall_time: 0.0008 s     Throughput: 0.016 GB/s
Benchmark: m_128_n_100_k_99_use_gpu_False        wall_time: 0.000261 s   Throughput: 0.049 GB/s
Benchmark: m_128_n_100_k_99_use_gpu_True         wall_time: 0.000794 s   Throughput: 0.0161 GB/s
Benchmark: m_128_n_100_k_100_use_gpu_False       wall_time: 0.000239 s   Throughput: 0.0536 GB/s
Benchmark: m_128_n_100_k_100_use_gpu_True        wall_time: 0.000777 s   Throughput: 0.0165 GB/s
Benchmark: m_128_n_1000_k_1_use_gpu_False        wall_time: 0.000324 s   Throughput: 0.395 GB/s
Benchmark: m_128_n_1000_k_1_use_gpu_True         wall_time: 0.000916 s   Throughput: 0.14 GB/s
Benchmark: m_128_n_1000_k_10_use_gpu_False       wall_time: 0.00042 s    Throughput: 0.305 GB/s
Benchmark: m_128_n_1000_k_10_use_gpu_True        wall_time: 0.000902 s   Throughput: 0.142 GB/s
Benchmark: m_128_n_1000_k_500_use_gpu_False      wall_time: 0.0011 s     Throughput: 0.116 GB/s
Benchmark: m_128_n_1000_k_500_use_gpu_True       wall_time: 0.00097 s    Throughput: 0.132 GB/s
Benchmark: m_128_n_1000_k_990_use_gpu_False      wall_time: 0.00133 s    Throughput: 0.0962 GB/s
Benchmark: m_128_n_1000_k_990_use_gpu_True       wall_time: 0.000993 s   Throughput: 0.129 GB/s
Benchmark: m_128_n_1000_k_1000_use_gpu_False     wall_time: 0.00102 s    Throughput: 0.126 GB/s
Benchmark: m_128_n_1000_k_1000_use_gpu_True      wall_time: 0.000964 s   Throughput: 0.133 GB/s
Benchmark: m_128_n_10000_k_10_use_gpu_False      wall_time: 0.002 s      Throughput: 0.64 GB/s
Benchmark: m_128_n_10000_k_10_use_gpu_True       wall_time: 0.00288 s    Throughput: 0.445 GB/s
Benchmark: m_128_n_10000_k_100_use_gpu_False     wall_time: 0.00233 s    Throughput: 0.549 GB/s
Benchmark: m_128_n_10000_k_100_use_gpu_True      wall_time: 0.00325 s    Throughput: 0.394 GB/s
Benchmark: m_128_n_10000_k_5000_use_gpu_False    wall_time: 0.0127 s     Throughput: 0.101 GB/s
Benchmark: m_128_n_10000_k_5000_use_gpu_True     wall_time: 0.00381 s    Throughput: 0.336 GB/s
Benchmark: m_128_n_10000_k_9900_use_gpu_False    wall_time: 0.015 s      Throughput: 0.0853 GB/s
Benchmark: m_128_n_10000_k_9900_use_gpu_True     wall_time: 0.00438 s    Throughput: 0.292 GB/s
Benchmark: m_128_n_10000_k_10000_use_gpu_False   wall_time: 0.0104 s     Throughput: 0.123 GB/s
Benchmark: m_128_n_10000_k_10000_use_gpu_True    wall_time: 0.00427 s    Throughput: 0.3 GB/s
Benchmark: m_128_n_100000_k_100_use_gpu_False    wall_time: 0.0148 s     Throughput: 0.865 GB/s
Benchmark: m_128_n_100000_k_100_use_gpu_True     wall_time: 0.0262 s     Throughput: 0.488 GB/s
Benchmark: m_128_n_100000_k_1000_use_gpu_False   wall_time: 0.0201 s     Throughput: 0.636 GB/s
Benchmark: m_128_n_100000_k_1000_use_gpu_True    wall_time: 0.0263 s     Throughput: 0.486 GB/s
Benchmark: m_128_n_100000_k_50000_use_gpu_False          wall_time: 0.214 s      Throughput: 0.0599 GB/s
Benchmark: m_128_n_100000_k_50000_use_gpu_True   wall_time: 0.0322 s     Throughput: 0.398 GB/s
Benchmark: m_128_n_100000_k_99000_use_gpu_False          wall_time: 0.262 s      Throughput: 0.0489 GB/s
Benchmark: m_128_n_100000_k_99000_use_gpu_True   wall_time: 0.0377 s     Throughput: 0.34 GB/s
Benchmark: m_128_n_100000_k_100000_use_gpu_False         wall_time: 0.118 s      Throughput: 0.108 GB/s
Benchmark: m_128_n_100000_k_100000_use_gpu_True          wall_time: 0.0365 s     Throughput: 0.351 GB/s

END_PUBLIC

BEGIN_PUBLIC
Automated g4 rollback of changelist 157169178

PiperOrigin-RevId: 161124193