Small bugfix in langevin sampler

[jwnys]

Bug fixes to Langevin sampler:

• when providing chunk_size != None, we got a tracing error
• for some models the grad function failed due to a shape (1,)

[codecov]

Codecov Report

Merging #1465 (7c5c0f8) into master (aa2f609) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1465   +/-   ##
=======================================
  Coverage   83.62%   83.62%           
=======================================
  Files         235      235           
  Lines       13542    13543    +1     
  Branches     2051     2052    +1     
=======================================
+ Hits        11325    11326    +1     
  Misses       1705     1705           
  Partials      512      512           

Impacted Files	Coverage Δ
netket/sampler/rules/langevin.py	90.69% <100.00%> (+0.22%)	⬆️

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[PhilipVinc]

Thanks!