Avoid unnecessary transaction in QuerySet.bulk_create

[Simon Charette]

The QuerySet.bulk_create method creates an unnecessary transaction when it can perform the insert in a singe INSERT when the following criteria are met

1. There's no mix of objects with and without primary key (this could potentially be optimized to be done in a single query but that's another can of worm)
2. The objects fit in a single batch with respect to the provided batch_size and connection.ops.bulk_batch_size

This is unnecessary overhead we've optimized in other areas of the code base (e.g. Model.save) which seems warranted in this case particularly because bulk_create offers a more featureful interface than create ​which can be used to insert a small number of rows.

[Sarah Boyce <42296566+sarahboyce@…>]

In e1671278:

Fixed #36490 -- Avoided unnecessary transaction in bulk_create.

When dealing with an heterogeneous set of object with regards to primary key
assignment that fits in a single batch there's no need to wrap the single
INSERT statement in a transaction.

[Sarah Boyce <42296566+sarahboyce@…>]

In 5eca562a:

Refs #36490 -- Simplified QuerySet._batched_insert returning fields handling.

Whether or not returning_fields should be specified to _insert is not a
function of each batches so the conditional can be moved outside of the loop.