fix: check payload before accumulating bytes received (#430)

Aggregators sometimes send empty messages at the end of training. Previously, the trainer would throw an error when attempting to get the number of bytes received from an empty message.
This is fixed by checking for an empty payload.

Author: GustavBaumgart

lib/python/flame/channel.py

@@ -229,8 +229,9 @@ async def _get():
             payload = None
             try:
                 payload = await self._ends[end_id].get()
-                # ignore timestamp for measuring bytes received
-                self.mc.accumulate("bytes", "recv", len(payload[0]))
+                if payload:
+                    # ignore timestamp for measuring bytes received
+                    self.mc.accumulate("bytes", "recv", len(payload[0]))
             except KeyError:
                 return None
 
@@ -345,8 +346,9 @@ async def _get_inner(end_id) -> tuple[str, Any]:
             payload = None
             try:
                 payload = await self._ends[end_id].get()
-                # ignore timestamp for measuring bytes received
-                self.mc.accumulate("bytes", "recv", len(payload[0]))
+                if payload:
+                    # ignore timestamp for measuring bytes received
+                    self.mc.accumulate("bytes", "recv", len(payload[0]))
             except KeyError:
                 yield end_id, None