[Operator] Add vstack op [MooreThreads]

[yjl0101]

add vstack operator
perf of some cases on NV A100:

benchmark/test_special_perf.py Operator vstack Performance Test (torch.float16)
Size        Torch Latency (ms)   Gems Latency (ms)
--------------------------------------------------
1024                  0.017408            0.011264
6144                  0.057344            0.032768
11264                  0.09728            0.053248
16384                 0.136192            0.070656
21504                 0.177152            0.090112
26624                 0.217088             0.10752
31744                 0.257024            0.125952
36864                 0.297984            0.144384
41984                 0.338944            0.162816
47104                 0.379904            0.181248
52224                 0.420864             0.19968
57344                 0.461824            0.218112
62464                 0.502784            0.236544
67584                 0.544768            0.254976
72704                 0.585728            0.273408
77824                 0.627712             0.29184
Operator vstack Performance Test (torch.float32)
Size        Torch Latency (ms)   Gems Latency (ms)
--------------------------------------------------
1024                  0.016384             0.01536
6144                  0.059392             0.05632
11264                 0.098304            0.093184
16384                 0.137216            0.130048
21504                 0.177152            0.166912
26624                 0.217088            0.203776
31744                 0.258048             0.24064
36864                 0.297984            0.277504
41984                 0.336896            0.314368
47104                 0.376832            0.351232
52224                 0.417792             0.38912
57344                 0.459776            0.425984
62464                 0.500736            0.462848
67584                 0.539648            0.499712
72704                 0.580608            0.535552
77824                  0.61952             0.57344
Operator vstack Performance Test (torch.bfloat16)
Size        Torch Latency (ms)   Gems Latency (ms)
--------------------------------------------------
1024                  0.017408            0.012288
6144                  0.058368            0.033792
11264                  0.09728            0.053248
16384                 0.137216             0.07168
21504                 0.177152            0.090112
26624                 0.217088            0.108544
31744                 0.257024            0.125952
36864                 0.297984            0.144384
41984                 0.338944            0.162816
47104                 0.379904            0.181248
52224                 0.420864             0.19968
57344                 0.461824            0.218112
62464                 0.503808            0.236544
67584                 0.544768            0.254976
72704                 0.585728            0.273408
77824                 0.627712             0.29184

[iclementine]

LGTM

[iclementine]

When the 4 tensors to be concatenated in this iteration have very varying number of rows, this grid may have many CTAs doing nothing. Do you have some test about the performance at this case? Maybe we can sort this tensors according to their number of rows.

Also, maybe this strategy is worth only when the number of tensors to vstack is large enough? But it is a good idea to take 4 tensors a time, compared to a naive one-by-one strategy.