Move observations to cpu in PyroConverter

[fonnesbeck]

Implements fix suggested in #1529 to allow traces on the GPU to be moved back to the CPU.

Closes #1529

[codecov]

Codecov Report

Merging #1659 (882e3bb) into main (f98c5fc) will not change coverage.
The diff coverage is n/a.

❗ Current head 882e3bb differs from pull request most recent head 6b76eb2. Consider uploading reports for the commit 6b76eb2 to get more accurate results

@@           Coverage Diff           @@
##             main    #1659   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files         108      108           
  Lines       11720    11720           
=======================================
  Hits        10646    10646           
  Misses       1074     1074           

Impacted Files	Coverage Δ
arviz/data/io_pyro.py	96.24% <ø> (ø)

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