Fix #1078

[PhilipVinc]

Lazily initialise the Tensorboardx's SummaryWriter

[github-actions]

Hello and thanks for your Contribution!
I will be building previews of the updated documentation at the following link:
https://netket.github.io/netket/preview/pv/log

Once the PR is closed or merged, the preview will be automatically deleted.

[codecov-commenter]

Codecov Report

Merging #1086 (e1c2d32) into master (88e964d) will decrease coverage by 0.02%.
The diff coverage is 14.28%.

❗ Current head e1c2d32 differs from pull request most recent head c09254b. Consider uploading reports for the commit c09254b to get more accurate results

@@            Coverage Diff             @@
##           master    #1086      +/-   ##
==========================================
- Coverage   82.04%   82.02%   -0.03%     
==========================================
  Files         204      204              
  Lines       12127    12132       +5     
  Branches     1813     1815       +2     
==========================================
+ Hits         9950     9951       +1     
- Misses       1743     1747       +4     
  Partials      434      434              

Impacted Files	Coverage Δ
netket/logging/tensorboard.py	22.22% <14.28%> (-0.23%)	⬇️

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