[Logging] Measure policy weight update

[allenwang28]

Not sure what issue @felipemello1 had earlier but I was able to see:

WandbBackend: Logged 92 metrics at step 2
=== [global_logger_5Dxs_r0] - METRICS STEP 2 ===
  buffer/add/count_episodes_added: 16.0
  ...
  policy_worker_perf/update/total_duration_avg_s: 6.04653369140625
  policy_worker_perf/update/total_duration_max_s: 6.04653369140625
  reference_perf/forward/avg_sequence_length: 1024.0
  reference_perf/forward/count_forward_passes: 2.0
==============================

[felipemello1]

good stuff!

I think i added this later thinking of workers that may crash/respawn after we call .init_backends, and it probably fixed this issue too. Glad that you tried!

https://github.com/meta-pytorch/forge/blob/1f27b54984bd2c79c82d2c07b9540e057aa81c23/src/forge/observability/metric_actors.py#L243