Summary:
There were two "bugs" associated with loading pickle files:

The first is easy: these files should have been `open()`ed as binary, but
weren't.

The second is slightly nuanced. The default encoding used wwhile unpickling
is 7-bit (ASCII.) However, the blobs are arbitrary 8-bit bytes which don't
agree. The absolute correct way to do this is to use `encoding="bytes"` and
then interpret the blob names either as ASCII, or better, as unicode utf-8. A
reasonable fix, however, is to treat it the encoding as 8-bit latin1 (which
agrees with the first 256 characters of Unicode anyway.)

As part of this, I also centralized all pickling operations into
`detectron.utils.io`.

This /still/ does not change the build to Python3, but I believe it is ready
now.

Reviewed By: rbgirshick

Differential Revision: D9689294

fbshipit-source-id: add1f2d784fe196df27b20e65e35922536d11a3c