Fix issue 108

[veugene]

Heya, I noticed that I hit issue #108 when I encountered its side-effect of causing my script to hang on exit. To avoid a hang, the flusher thread no longer joins itself! However, since the thread is a daemon, some weakref.finalizer magic is used to wait for a clean shutdown of the thread and then have the event writer flush to disk, on exit. The finalizer uses a weak reference to the event writer so as to not prevent the writer (and thread) from being garbage collected before the end of the script.

Issue #108 shows that event files are not closed on exit. This is fixed here.

[veugene]

Works for python 3 but the weakref API for python 2 doesn't have finalize. :(

It would be a shame to have to use a much uglier atexit hack to make this work with python 2. How much longer will this library support python 2?

[codecov-io]

Codecov Report

Merging #498 into master will increase coverage by 0.01%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #498      +/-   ##
==========================================
+ Coverage   89.01%   89.03%   +0.01%     
==========================================
  Files          37       37              
  Lines        2576     2589      +13     
==========================================
+ Hits         2293     2305      +12     
- Misses        283      284       +1

Impacted Files	Coverage Δ
tensorboardX/event_file_writer.py	94.91% <94.73%> (-0.33%)	⬇️

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