Downgrade docker images to use Python 3.8

[abidwael]

In #3320, we update the docker images to use Python 3.9.

In #3324, we update the type hinting to match what's accepted in Python 3.9, we see the following failure in the CI (failures for torchscript tests). By following the torchscript typing guide, it looks like only python 3.8-style type hints are accepted.

In this PR, we use Python 3.8 in the docker images.

[github-actions]

Unit Test Results

  4 files  ±0    4 suites  ±0   27m 24s ⏱️ - 1m 19s
33 tests ±0  30 ✔️ ±0  3 💤 ±0  0 ❌ ±0 
53 runs  ±0  46 ✔️ ±0  7 💤 ±0  0 ❌ ±0 

Results for commit 774e7ce. ± Comparison against base commit e49eae9.