[prototype] Speed improvement for adjust gamma op

[datumbox]

[---------------- Adjust_gamma cpu torch.uint8 ---------------]
                     |  adjust_gamma stable  |  adjust_gamma v2
1 threads: ----------------------------------------------------
      (3, 400, 400)  |          1300         |         828     
6 threads: ----------------------------------------------------
      (3, 400, 400)  |          1610         |        1070     

Times are in microseconds (us).

[--------------- Adjust_gamma cuda torch.uint8 ---------------]
                     |  adjust_gamma stable  |  adjust_gamma v2
1 threads: ----------------------------------------------------
      (3, 400, 400)  |          83.2         |        41.7     
6 threads: ----------------------------------------------------
      (3, 400, 400)  |          82.8         |        41.7     

Times are in microseconds (us).

[--------------- Adjust_gamma cpu torch.float32 --------------]
                     |  adjust_gamma stable  |  adjust_gamma v2
1 threads: ----------------------------------------------------
      (3, 400, 400)  |          8.09         |        7.74     
6 threads: ----------------------------------------------------
      (3, 400, 400)  |          6.30         |        5.87     

Times are in milliseconds (ms).

[-------------- Adjust_gamma cuda torch.float32 --------------]
                     |  adjust_gamma stable  |  adjust_gamma v2
1 threads: ----------------------------------------------------
      (3, 400, 400)  |          30.2         |        9.0      
6 threads: ----------------------------------------------------
      (3, 400, 400)  |          30.2         |        8.8      

Times are in microseconds (us).

Benchmark script from here

cc @vfdev-5 @bjuncek @pmeier

[vfdev-5]

Lgtm, thanks @datumbox

[datumbox]

The failures on windows are unrelated. It seems a Datapipes issue. cc @pmeier