Phase 5: asyncio.TaskGroup and modern async patterns (#4878)

Summary:
Pull Request resolved: #4878

Migrate to modern Python 3.11+ async patterns using asyncio.TaskGroup for
structured concurrency and fix deprecated asyncio usage.

Files changed:
- ax/fb/runners/chronos.py: Convert run_concurrently() from asyncio.gather(*tasks)
  to asyncio.TaskGroup for better exception handling and structured concurrency.
- ax/fb/storage/external_store/hdf5_utils.py: Replace deprecated
  asyncio.get_event_loop() + run_until_complete() pattern with asyncio.run() and
  asyncio.TaskGroup.
- ax/utils/common/executils.py: Fix missing await before asyncio.sleep() that was
  causing the sleep to have no effect (was flagged with pyre-fixme[1001]).

Reviewed By: mpolson64

Differential Revision: D91648881

fbshipit-source-id: 840372cb1aa83a4472a7cfc850abb5af2b7e8882

Author: Balandat

ax/utils/common/executils.py

@@ -121,9 +121,7 @@ async def async_actual_wrapper(*args, **kwargs):
                             wait_interval = min(
                                 MAX_WAIT_SECONDS, initial_wait_seconds * 2 ** (i - 1)
                             )
-                            # pyre-fixme[1001]: `asyncio.sleep(wait_interval)` is
-                            #  never awaited.
-                            asyncio.sleep(wait_interval)
+                            await asyncio.sleep(wait_interval)
                         return await func(*args, **kwargs)
                 # If we are here, it means the retries were finished but
                 # The error was suppressed. Hence return the default value provided.