fix getPingLatency in pt2pt mode

Summary:
D52715647 incorrectly used P2POp in blocking `getPingLatency`/`getPingPongLatency`. P2POp enqueues ops for batch_isend_recv, and need additional call to batch_isend_recv to kick off enqueued ops. Thus, the previous version caused incorrect  latency, since we just enqueued but didn't issue.

This patch fixes it by using send/recv API.

Reviewed By: kingchc

Differential Revision: D52721469

fbshipit-source-id: 9fd44654f1fc97729ff9c333257b59938163f259

train/comms/pt/comms.py

@@ -630,17 +630,15 @@ def getPingLatency(self, numIters):
                     self.collectiveArgs.global_rank
                 )
                 self.collectiveArgs.dst_rank = self.collectiveArgs.dst_ranks[idx]
-                self.collectiveArgs.collective = "send"
-                self.backendFuncs.P2POp(
+                self.backendFuncs.send(
                     collectiveArgs=self.collectiveArgs,
                 )
             elif self.collectiveArgs.global_rank in self.collectiveArgs.dst_ranks:
                 idx = self.collectiveArgs.dst_ranks.index(
                     self.collectiveArgs.global_rank
                 )
                 self.collectiveArgs.src_rank = self.collectiveArgs.src_ranks[idx]
-                self.collectiveArgs.collective = "recv"
-                self.backendFuncs.P2POp(
+                self.backendFuncs.recv(
                     collectiveArgs=self.collectiveArgs,
                 )
             self.backendFuncs.complete_accel_ops(self.collectiveArgs)
@@ -666,27 +664,23 @@ def getPingPongLatency(self, numIters):
                     self.collectiveArgs.global_rank
                 )
                 self.collectiveArgs.dst_rank = self.collectiveArgs.dst_ranks[idx]
-                self.collectiveArgs.collective = "send"
-                self.backendFuncs.P2POp(
+                self.backendFuncs.send(
                     collectiveArgs=self.collectiveArgs,
                 )
                 self.collectiveArgs.src_rank = self.collectiveArgs.dst_ranks[idx]
-                self.collectiveArgs.collective = "recv"
-                self.backendFuncs.P2POp(
+                self.backendFuncs.recv(
                     collectiveArgs=self.collectiveArgs,
                 )
             elif self.collectiveArgs.global_rank in self.collectiveArgs.dst_ranks:
                 idx = self.collectiveArgs.dst_ranks.index(
                     self.collectiveArgs.global_rank
                 )
                 self.collectiveArgs.src_rank = self.collectiveArgs.src_ranks[idx]
-                self.collectiveArgs.collective = "recv"
-                self.backendFuncs.P2POp(
+                self.backendFuncs.recv(
                     collectiveArgs=self.collectiveArgs,
                 )
                 self.collectiveArgs.dst_rank = self.collectiveArgs.src_ranks[idx]
-                self.collectiveArgs.collective = "send"
-                self.backendFuncs.P2POp(
+                self.backendFuncs.send(
                     collectiveArgs=self.collectiveArgs,
                 )
             self.backendFuncs.complete_accel_ops(self.collectiveArgs)