[tune] Reduce the times for flushing json object to file (#4198)

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## What do these changes do?

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When we write one result using JsonLogger, it will call 'flush' many times, which may cost a lot of time when writing  to a remote distributed filesystem.

## Related issue number
#4197 
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python/ray/tune/logger.py

@@ -90,10 +90,10 @@ class JsonLogger(Logger):
     def on_result(self, result):
         json.dump(result, self, cls=_SafeFallbackEncoder)
         self.write("\n")
+        self.local_out.flush()
 
     def write(self, b):
         self.local_out.write(b)
-        self.local_out.flush()
 
     def flush(self):
         self.local_out.flush()