- Add environment variables to prevent over-allocation of GPU VRAM
  by TensorFlow and/or Jax.
- Add 'py{3x}-ci' test environments that are tweaked specifically
  for our CI runner, adding instructions that offer as much support
  for GPU as possible given our runner host's configuration.
- At the moment, an update of the host to the latest CUDA version
  enables native support for Torch, and TensorFlow with Python>=3.9
  (but not 3.8), plus Jax with some extra installation command.

- Add the 'scripts/run_tests.sh' bash script, which enables running
  various blocks of tests. This mostly refactors instructions that
  used to be part of 'tox.ini', in a more convenient way:
  - some tests are grouped (commands in a group _all_ run, but the
    group fails if any has failed once all are done);
  - instructions are (hopefully) easier to read through and should
    be easier to maintain in the future;
  - it should be easier to decline more groups in the future (e.g.
    to have a routine with only unit tests);
  - groups of tests can be run easily, including without using tox.
- Have the 'tox.ini' file trigger some groups of tests by calling
  the new bash script. This makes the tox file easier to read and
  parse through, and merely oriented at providing run isolation.
- Split test groups into various envs in tox, which may be slightly
  more costly to run, but enables writing simpler CI/CD pipelines
  that merely pass arguments to the tox command.
- Make it explicit in 'tox.ini' that tests can be run with python
  3.8 and onwards.
- Keep a "main" job that groups all others. This is useful in the
  Inria shared Gitlab Runner, as caching is disabled, so that the
  use of distinct jobs would result in redundant costful package
  installations.