Multigrid training

Thank you for releasing such very useful tool.

After reading "A Multigrid Method for Efficiently Training Video Models", I am a bit confused about the multigrid scheduling policy. In the paper, it is mentioned 4 approaches: baseline, long cycles, short cycles, long +short cycles (default setting)


However, after displaying the multigrid policy used in this repo, I get:

```
[2020-06-25 16:51:23,077] PID : 28742 : INFO : Long cycle index Base shape Epochs
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 0 [8, 8, 158] 73
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 0 [4, 16, 158] 110
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 0 [2, 16, 224] 142
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 0 [1, 32, 224] 158
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 1 [8, 8, 158] 205
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 1 [4, 16, 158] 228
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 1 [2, 16, 224] 248
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 1 [1, 32, 224] 259
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 2 [8, 8, 158] 291
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 2 [4, 16, 158] 308
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 2 [2, 16, 224] 322
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 2 [1, 32, 224] 329
[2020-06-25 16:51:23,078] PID : 28742 : INFO : 3 [1, 32, 224] 358
[2020-06-25 16:51:23,078] PID : 28742 : INFO : Long cycle updates:
[2020-06-25 16:51:23,078] PID : 28742 : INFO : BN.NORM_TYPE: sub_batchnorm
[2020-06-25 16:51:23,078] PID : 28742 : INFO : BN.NUM_SPLITS: 8
[2020-06-25 16:51:23,078] PID : 28742 : INFO : TRAIN.BATCH_SIZE: 512
[2020-06-25 16:51:23,078] PID : 28742 : INFO : DATA.NUM_FRAMES x LONG_CYCLE_SAMPLING_RATE: 8x8
[2020-06-25 16:51:23,078] PID : 28742 : INFO : DATA.TRAIN_CROP_SIZE: 158
```

In the paper, it is stated that: