[tune] placement group support (#13370)

doc/source/tune/user-guide.rst

@@ -688,6 +688,8 @@ These are the environment variables Ray Tune currently considers:
   experiment state is checkpointed. If not set this will default to ``10``.
 * **TUNE_MAX_LEN_IDENTIFIER**: Maximum length of trial subdirectory names (those
   with the parameter values in them)
+* **TUNE_MAX_PENDING_TRIALS_PG**: Maximum number of pending trials when placement groups are used. Defaults
+  to ``1000``.
 * **TUNE_RESULT_DIR**: Directory where Ray Tune trial results are stored. If this
   is not set, ``~/ray_results`` will be used.
 * **TUNE_RESULT_BUFFER_LENGTH**: Ray Tune can buffer results from trainables before they are passed
@@ -697,6 +699,9 @@ These are the environment variables Ray Tune currently considers:
   but never longer than this value. Defaults to 100 (seconds).
 * **TUNE_RESULT_BUFFER_MIN_TIME_S**: Additionally, you can specify a minimum time to buffer results. Defaults to 0.
 * **TUNE_SYNCER_VERBOSITY**: Amount of command output when using Tune with Docker Syncer. Defaults to 0.
+* **TUNE_TRIAL_STARTUP_GRACE_PERIOD**: Amount of time after starting a trial that Ray Tune checks for successful
+  trial startups. After the grace period, Tune will block until a result from a running trial is received. Can
+  be disabled by setting this to lower or equal to 0.
 * **TUNE_WARN_THRESHOLD_S**: Threshold for logging if an Tune event loop operation takes too long. Defaults to 0.5 (seconds).
 * **TUNE_STATE_REFRESH_PERIOD**: Frequency of updating the resource tracking from Ray. Defaults to 10 (seconds).