[RLlib] Include SampleBatch.T column in all collected batches

[ArturNiederfahrenhorst]

Why are these changes needed?

Most view requirements fall short of the SampleBatch.T='t' field, which is not accessed in dummy batch passes of models.
Therefore, this information is excluded from collected sample batches by default.
In most setups, this is not of interest. But it must be included when saving batches for offline learning purposes and for timeslicing of MultiAgentBatches. Our implementation of timeslice() build on availability of the column SampleBatch.T.

This small PR paves the way for my other PR, which needs this information for the replay buffer lockstep mode.

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 :(

[sven1977]

Looks great. Thanks for unifying this.