Various cleanups

[akx]

This PR:

• cleans up requirements (could still clean up more; e.g. inference shouldn't require all of the things training does)
• fixes some minor bugs (e.g. leaky globals, this and that)
• makes the project work even without bitsandbytes
• deduplicates some code

[akx]

NB: I haven't run this branch on Valohai yet (I don't have access to the project/datasets/S3 bucket currently used), so if someone wants to give that a shot...

[tokkoro]

Changes are clean and clear.

Was not able to properly run executions as I don't have access to the data store.

[SofiaChar]

Looking good

[tokkoro]

@akx can you rebase so it can be merged? :)

[akx]

@tokkoro Done. Rebasing took a while since I had to untangle some of the duplicate work done in a4b5451.