Checkpoint nemo2 migration

[Marlon666]

Update checkpoint log pattern matching to support NeMo 2's
'Global Checkpoint Save' message format, which provides epoch
timestamps and save duration directly in the log message.

Changes:

• log_patterns.py: Update CHECKPOINT_WRITE_START regex to match
  NeMo 2's 'Global Checkpoint Save : Rank: X : Iteration: N :
  Start time: EPOCHs : Save duration: Xs' format
• calculate_checkpoint_metrics.py: Extract start time as epoch
  float from the message instead of parsing log prefix timestamp;
  map end message step (N+1) to start iteration (N)
• calculate_checkpoint_metrics_test.py: Add 14 unit tests covering
  pattern matching, timestamp parsing, checkpoint extraction, and
  write duration computation

docs: update checkpointing metrics README for NeMo 2

• Specify NeMo 2 compatibility
• Add sample output from a real NeMo 2 run
• Add testing section with unittest command
• Fix stray backtick in pip install command

How to add a new framework

1. Create <framework>_parser.py, subclass LogParser
2. Decorate with @register_parser
3. Import in calculate_checkpoint_metrics.py
4. Add tests — no core logic changes needed

Tested:

> python calculate_checkpoint_metrics.py \
    --gcs_logs_path gs://tess-benchmark-outputs/muzi-8b-dl-ckpt-20260217-175559
Analyzing file: muzi-8b-dl-ckpt-20260217-175559/nemo_log_globalrank-1_localrank-1.txt, Global rank: 1, Local rank: 1
Auto-detected log format: nemo2
Analyzing file: muzi-8b-dl-ckpt-20260217-175559/run_0/nemo_log_globalrank-2_localrank-2.txt, Global rank: 2, Local rank: 2
Auto-detected log format: nemo2
Analyzing file: muzi-8b-dl-ckpt-20260217-175559/nemo_log_globalrank-0_localrank-0.txt, Global rank: 0, Local rank: 0
Auto-detected log format: nemo2
Analyzing file: muzi-8b-dl-ckpt-20260217-175559/nemo_log_globalrank-3_localrank-3.txt, Global rank: 3, Local rank: 3
Auto-detected log format: nemo2
min checkpoint write duration: 4.9700000286102295s
max checkpoint write duration: 34.86500000953674s
average checkpoint write duration: 17.880999982357025s
checkpoint write time standard deviation: 12.443055009810607