update to latest mmcv and fixed training

[SebastienLinker]

It works with MMCV 1.3.3, and training st_gcn_aaai18/kinetics-skeleton (did not test other configs)

Regarding the loss function, latest mmcv does not propagate it, so I had to use a global variable

Thanks to #311 for the solution on training accuracy Also includes #412

Note: Might need to install requirements.txt twice, first with pytorch and pycocotools, then install mmcv and mmdet