Tracking issue - batch mode errors

[rg936672]

What's the issue?

Batch mode is currently quite buggy and under-tested. This is a tracking issue to help keep track of batch mode bugs when they crop up.

• Batch mode fails unless batch_size is the length of the training data #265
• The VariationalInference decorator has an issue with the use of batch_size #377

We're not yet convinced that batch mode is fully tested. Maybe we should mark it as experimental?