REF: Move minibatch selection into solvers

[carterbox]

Purpose

Make unique minibatches each epoch instead of using the same minibatches each epoch.

Approach

Move minibatch selection into solvers so that indexing is used to select the minibatches each epoch instead of separating minibatches into a list when the data is pushed to the GPU(s).

No new functionality that needs testing.

Pre-Merge Checklists

Submitter

• [x] Write a helpfully descriptive pull request title.
• [x] Organize changes into logically grouped commits with descriptive commit messages.
• [x] Document all new functions.
• [x] Write tests for new functions or explain why they are not needed.
• [ ] Build the documentation successfully
• [x] Use yapf to format python code.

Reviewer

• [ ] Actually read all of the code.
• [ ] Run the new code yourself.
• [ ] Write a summary of the changes as you understand them.
• [ ] Thank the submitter.