Adds version_base to launch

[rsokl]

This mirrors functionality that will be exposed by hydra.run and hydra.initialize starting with Hydra 1.2.0:

• If the version_base parameter is not specified, Hydra 1.x will use defaults compatible with version 1.1. Also in this case, a warning is issued to indicate an explicit version_base is preferred.
• If the version_base parameter is None, then the defaults are chosen for the current minor Hydra version. For example for Hydra 1.2, then would imply config_path=None and hydra.job.chdir=False.
• If the version_base parameter is an explicit version string like "1.1", then the defaults appropriate to that version are used.

Our tests will specify version_base in accordance with the local version of Hydra, unless otherwise appropriate, via a pytest fixture.