[Core] GC Debug callback (#24829)

Signed-off-by: Jialin Ouyang <jialino@meta.com>
Signed-off-by: Jialin Ouyang <Jialin.Ouyang@gmail.com>
Co-authored-by: Jialin Ouyang <jialino@meta.com>

docs/contributing/profiling.md

@@ -208,3 +208,11 @@ One example is [snakeviz](https://jiffyclub.github.io/snakeviz/).
 pip install snakeviz
 snakeviz expensive_function.prof
 ```
+
+### Analyzing Garbage Collection Costs
+
+Leverage VLLM_GC_DEBUG environment variable to debug GC costs.
+
+- VLLM_GC_DEBUG=1: enable GC debugger with gc.collect elpased times
+- VLLM_GC_DEBUG='{"top_objects":5}': enable GC debugger to log top 5
+  collected objects for each gc.collect