Support garbage collection after pt2 compilation (#143364)

Summary:
**Context:**
recently we observed ~10% Training GPU memory regression, due to the not efficient recycling of the memory at Pytorch2 compilation time. This diff is to save the memory regression caused by the PT2 compilation.
Detailed debugging notes: https://docs.google.com/document/d/1EPopAyYyXwTnkyVaUJ5Xa_Uw9iWv3zimK7FkagKsKIY/edit?tab=t.0#bookmark=id.e5b26tcdfl5g

In this diff, we support garbage collection after pt2 compilation.

**Rollout / rollback plan:**
To ensure the system reliability, we design 2 layers of control for this change's rollout:
- Add jk to control the global rollout / rollback of this functionality. The jk is on by default
- Add env var to control individual job's rollout. The env var is on by default.

X-link: pytorch/pytorch#143364
Approved by: https://github.com/ezyang

Reviewed By: ezyang

Differential Revision: D67328568

Pulled By: huydhn

fbshipit-source-id: d0c856846bef3bdd3b060df90cf5888d57245ff8

userbenchmark/dynamo/dynamobench/_dynamo/utils.py

@@ -917,6 +917,7 @@ class CompilationMetrics:
     feature_usage: Optional[dict[str, bool]] = None
     compile_time_autotune_time_us: Optional[int] = None
     is_runtime: Optional[bool] = False
+    gc_time_us: Optional[int] = None
 
 
 DEFAULT_COMPILATION_METRICS_LIMIT = 64