[Feature] microbatches should default run in parallel according to threads

[graciegoheen]

Describe the feature

For faster processing of microbatch models, we should by default run the batches in parallel. We are assuming batches can be run independently unless the adapter doesn't support it.

We could additionally:

• raise an exception when the adapter can't run batches in parallel but the microbatch model is configured as such
• have a config to optionally run batches sequentially for specific use cases
• automatically detect which batches can not be run in parallel

Slack thread