[RLlib] `Algorithm.add_policy()` should alternatively accept an already instantiated policy object.

[sven1977]

Signed-off-by: sven1977 [email protected]

Algorithm.add_policy() should alternatively accept an already instantiated policy object.

• Same for RolloutWorker.add_policy().
• Enhanced existing test case to cover this behavior.

Why are these changes needed?

Related issue number

Checks

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• [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 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 :(