text: Title: Tensorboard crashes when a scalar is added to a histogram
  Body: `tensorboardX==1.6.0`
  
  See title, eg tensorboard will report:
  ```
  ValueError: can only convert an array of size 1 to a Python scalar
  E0117 23:32:28.772649 Thread-1 _internal.py:88] Error on request:
  Traceback (most recent call last):
    File "/usr/local/lib/python3.7/site-packages/werkzeug/serving.py", line 270, in run_wsgi
      execute(self.server.app)
    File "/usr/local/lib/python3.7/site-packages/werkzeug/serving.py", line 258, in execute
      application_iter = app(environ, start_response)
    File "/usr/local/lib/python3.7/site-packages/tensorboard/backend/application.py", line 307, in __call__
      return self.data_applications[clean_path](environ, start_response)
    File "/usr/local/lib/python3.7/site-packages/werkzeug/wrappers.py", line 308, in application
      resp = f(*args[:-2] + (request,))
    File "/usr/local/lib/python3.7/site-packages/tensorboard/plugins/scalar/scalars_plugin.py", line 199, in scalars_route
      (body, mime_type) = self.scalars_impl(tag, run, experiment, output_format)
    File "/usr/local/lib/python3.7/site-packages/tensorboard/plugins/scalar/scalars_plugin.py", line 161, in scalars_impl
      for tensor_event in tensor_events]
    File "/usr/local/lib/python3.7/site-packages/tensorboard/plugins/scalar/scalars_plugin.py", line 161, in <listcomp>
      for tensor_event in tensor_events]
  ValueError: can only convert an array of size 1 to a Python scalar
  ```