[RLlib] DQN Rainbow on new API stack (w/ EnvRunner):`training_step` implementation.

[simonsays1980]

Why are these changes needed?

We are moving the standard algorithms to our new stack (i.e. RLModule API and EnvRunner API). This PR is one part of moving DQN Rainbow into our new stack. With it comes a training step that enables using the ÈnvRunner API together with RLModule.

See #43196 for the corresponding learners for DQN Rainbow.

Related issue number

Closes #37777

Checks

• [x] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [x] 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 added any new APIs to the API Reference. For example, if I added a method in Tune, I've added it in doc/source/tune/api/ under the corresponding .rst file.
• [x] 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
  • [x] Unit tests
  • [ ] Release tests
  • [ ] This PR is not tested :(