Summary:
Before this change <code>assert_and_infer_cfg</code> would override the weights passed by the user with CFG ones.

It is normal to change it as the user is required to pass weights, otherwise there will be this error :
<code>AssertionError: <class 'NoneType'></code>. (verified only in infer_simple.py)

Sort of follow-up of eddb1301.
Closes https://github.com/facebookresearch/Detectron/pull/331

Reviewed By: ir413

Differential Revision: D7569224

Pulled By: rbgirshick

fbshipit-source-id: 00dbaadd2440f39b2b3440b4101a58ed3e68a908