Fix Hydra warning by sticking to OmegaConf 2.0.x

[odelalleau]

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and Context / Related issue

The deprecation of OmegaConf.is_none() in OmegaConf 2.1 causes a warning
with Hydra 1.0.x. The fix consists in using an older version of
OmegaConf.

How Has This Been Tested (if it applies)

python -m pytest tests/core
python -m pytest tests/algorithms

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document and completed the CLA (see CONTRIBUTING).
• All tests passed, and additional code has been covered with new tests.