Lazily create summary writer for TF2 logger. (#5631)

Author: llan-ml

python/ray/tune/logger.py

@@ -154,11 +154,13 @@ def tf2_compat_logger(config, logdir):
 
 class TF2Logger(Logger):
     def _init(self):
-        from tensorflow.python.eager import context
-        self._context = context
-        self._file_writer = tf.summary.create_file_writer(self.logdir)
+        self._file_writer = None
 
     def on_result(self, result):
+        if self._file_writer is None:
+            from tensorflow.python.eager import context
+            self._context = context
+            self._file_writer = tf.summary.create_file_writer(self.logdir)
         with tf.device("/CPU:0"), self._context.eager_mode():
             with tf.summary.record_if(True), self._file_writer.as_default():
                 step = result.get(
@@ -181,10 +183,12 @@ def on_result(self, result):
         self._file_writer.flush()
 
     def flush(self):
-        self._file_writer.flush()
+        if self._file_writer is not None:
+            self._file_writer.flush()
 
     def close(self):
-        self._file_writer.close()
+        if self._file_writer is not None:
+            self._file_writer.close()
 
 
 def to_tf_values(result, path):