Fix mmcv submodule

[remmel]

Hello,

The mmcv submodule does not point to any repo. I suggest to fix it; and thus slighly simplify the setup process.

before:

$ git submodule status
fatal: no submodule mapping found in .gitmodules for path 'thirdparty/mmcv'

after:

$ git submodule status
 3ba02dfd35c326306346bd9d35a887efc2b30fb0 thirdparty/mmcv (v2.2.0-13-g3ba02dfd)

(Note that when changing branch, it might be able to run git submodule update --init because git clone --recursive does not get submodule of others branchs)

[lizhan17]

Thank you !