[RLlib] Moving sampling coordination for `batch_mode=complete_episodes` to `synchronous_parallel_sample`.

[simonsays1980]

Why are these changes needed

When sampling complete episodes each EnvRunner sampled train_batch_size before returning. This made sampling inefficient and led to long waiting times in case slow environments were used. Furthermore, scaling could not reduce the workload in sampling. This PR changes this and moves coordination of the sampling when complete_episodes are needed fully to synchronous_parallel_sample that can coordinate better across all EnvRunners. This should reduce sampling duration linearly by the number of EnvRunners chosen.

Related issue number

Closes #45826

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.
• [x] 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
  • [x] Release tests
  • [ ] This PR is not tested :(