removed special-casing of batch_size == 1 in various places, though not all

A quick fix, since this has caused problems before. I didn't add the special-casing, so not sure yet if this will break tests.

Left one special-case intact, since I wasn't sure what it did. See comment on line 1647.
This PR was submitted in response to Dustin Webb's post to pylearn-dev on 2014/06/16 13:37 EST, where he reports crashes when using batch_size == 1. I haven't had the time to recreate his problem myself, so unfortunately I can't be more specific than the crash stack trace he provided.