Summary:
local_variables makes the code in train_step really hard to read. Killing it
from all hooks will take time, so start from a single hook (on_step).

Differential Revision: D20171981

fbshipit-source-id: 79342642cbac9a8ebcc9ca59a2b7cce8d4f64f14

Summary:
This is part of a series of diffs to eliminate local_variables (see D20171981).
Now that we've removed local_variables from step, remove it
from train_step, eval_step.

Differential Revision: D20170006

fbshipit-source-id: 3d044c9e383601662ed19c05bc05b1ee23667709