[RLlib] CQL: Bug fixes and OPE example added to test and offline_rl.py example.

[sven1977]

CQL: OPE example via learnt Q-function added to test and offline_rl.py example.

• Fixed missing stats bug (stats_fn was not called anymore).
• Fixed bug in offline_rl.py example script.

Why are these changes needed?

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(