Add detailed hetero MIMO timeline profiling

[yashaswikarnati]

Summary

• add iteration-window and per-event flushing controls to hetero timeline tracing
• add memory, optimizer, grad-finalize, MIMO forward, and bridge communicator timeline events
• keep tracing inactive/no-op unless timeline profiling is enabled

Testing

• python3 -m py_compile examples/mimo/training/hetero/args.py examples/mimo/training/hetero/grad_sync.py examples/mimo/training/hetero/loop.py examples/mimo/training/hetero/step.py examples/mimo/training/hetero/timeline.py megatron/core/models/mimo/model/base.py megatron/core/pipeline_parallel/bridge_communicator.py megatron/core/pipeline_parallel/timeline.py
• git diff --check --cached
• pre-commit hooks during git commit/push: black, pylint, isort

[yashaswikarnati]

do we need this?

[yashaswikarnati]

Removed it. This was only a hang-forensics escape hatch for flushing an event-start record before every traced event, and it is not needed for the steady-state 1F1B timeline profiling path. The PR now keeps normal buffered flush behavior only.