Add internal_api.global_gc() method, which triggers gc.collect() on all workers (#7327)

python/ray/includes/libcoreworker.pxd

@@ -97,6 +97,7 @@ cdef extern from "ray/core_worker/core_worker.h" nogil:
                         c_vector[shared_ptr[CRayObject]] *returns,
                         const CWorkerID &worker_id) nogil,
                     CRayStatus() nogil,
+                    void() nogil,
                     c_bool ref_counting_enabled)
         CWorkerType &GetWorkerType()
         CLanguage &GetLanguage()
@@ -173,6 +174,7 @@ cdef extern from "ray/core_worker/core_worker.h" nogil:
                         int64_t timeout_ms, c_vector[c_bool] *results)
         CRayStatus Delete(const c_vector[CObjectID] &object_ids,
                           c_bool local_only, c_bool delete_creating_tasks)
+        CRayStatus TriggerGlobalGC()
         c_string MemoryUsageString()
 
         CWorkerContext &GetWorkerContext()