[RLlib] - Unify namings for actor managers' outstanding in-flight requests metrics.

[simonsays1980]

Why are these changes needed?

Right now we have three different namings for the same metric, locations are here:

• ray/rllib/algorithms/algorithm.py

  Line 3124 in 8b26f5c

  "num_in_flight_async_reqs"

• ray/rllib/algorithms/algorithm.py

  Line 3248 in 8b26f5c

  "actor_manager_num_outstanding_async_reqs": (

• ray/rllib/algorithms/algorithm.py

  Line 3697 in 8b26f5c

  "num_in_flight_async_sample_reqs"

This PR proposes to unify the namings to the one used in the LearnerGroup here, namely actor_manager_num_outstanding_async_reqs.

Related issue number

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